The EPPlus 6 Nuget package will no longer target .NET 4.0 and 4.5, as these versions are no longer supported.
EPPlus 6 will instead target .NET 4.5.2. A target for .NET 6 has also been added.

In EPPlus 6 all public interfaces referencing to System.Drawing.Common has been removed. The reason for this is that Microsoft has deprecated System.Drawing.Common on all non-Windows platforms. To replace System.Drawing.Common we have implemented native code to handle the different image formats and text measurings. To handle Colors EPPlus adds a reference to System.Drawing.Primitives. This means that all references to Color will work as before.

The following methods and properties have been removed or changed:

• ExcelDrawings.AddPicture(string, Image) - Has been removed. Use other overloads.
• ExcelDrawings.AddPicture(string, Image, Uri) - Has been removed. Use other overloads.
• ExcelDrawings.AddPictureAsync(string, Image) - Has been removed. Use other overloads.
• ExcelDrawings.AddPictureAsync(string, Image, Uri) - Has been removed.
• ExcelPicture.ImageFormat - Has been removed. Use other overloads.
• ExcelBackgroundImage.Image - Property has changed data type to ExcelImage. See description on ExcelImage below.
  ExcelBackgroundImage.Image can no longer be set to null. Use ExcelBackgroundImage.Remove instead.
• ExcelDrawingBlipFill.Image - Property has changed data type to ExcelImage. See description on ExcelImage below.
• ExcelDrawingBlipFill.ImageFormat - Has been removed. Use ExcelImage.Type instead.
• ExcelVmlDrawingPicture.Image - Property has changed data type to ExcelImage. See description on ExcelImage below.
• ExcelVmlDrawingPicture.InsertPicture - Has been removed.
• ExcelVmlDrawingPictureFill.Image - Property has changed data type to ExcelImage. See description on ExcelImage below.
• ExcelFont.SetFromFont(Font) - Has been replaced with new signature.
• ExcelTextFont.SetFromFont(Font) - Has been replaced with new signature.
• ExcelFontXml.SetFromFont(Font) - Has been replaced with new signature.

The ExcelImage class replaces all System.Drawing.Common.Image properties (see list above).

All SetImage and SetImageAsync methods returns the ExcelImage object. This object can be used with SetImage(ExcelImage) overload.

The internal image handler supports the following image formats:

• Bmp
• Jpeg/Exif
• Gif
• Png
• Tif
• Ico
• Svg
• WebP
• Wmf/Wmz
• Emf/Wmz

Switching to overloads supported natively by EPPlus 6 is recommended, but if you have a lot of references to System.Drawing.Common, adding extension methods for images and fonts might simplify...

using System;
using System.Drawing;
using System.Drawing.Imaging;
using System.IO;
using OfficeOpenXml.Drawing;
using OfficeOpenXml.Style;
namespace OfficeOpenXml.Drawing.Extensions
{
    public static class EPPlusDrawingExtensions
    {
        /// <summary>
        /// Adds a picture to the drawings 
        /// </summary>
        /// <param name="drawings">The collection </param>
        /// <param name="name">The name of the drawing</param>
        /// <param name="image">The image object to add</param>
        /// <returns>A picture added to the drawings collection.</returns>
        public static ExcelPicture AddPicture(this ExcelDrawings drawings, string name, Image image)
        {
            var ms = new MemoryStream();
            image.Save(ms, image.RawFormat);
            return drawings.AddPicture(name, ms, GetPictureType(image));
        }
        /// <summary>
        /// Adds a picture to the drawings 
        /// </summary>
        /// <param name="drawings">The collection </param>
        /// <param name="name">The name of the drawing</param>
        /// <param name="image">The image object to add</param>
        /// <param name="uri">An hyperlink reference used when clicking the drawing object.</param>
        /// <returns>A picture added to the drawings collection.</returns>
        public static ExcelPicture AddPicture(this ExcelDrawings drawings, string name, Image image, Uri uri)
        {
            var ms = new MemoryStream();
            image.Save(ms, image.RawFormat);
            return drawings.AddPicture(name, ms, GetPictureType(image), uri);
        }
        /// <summary>
        /// Sets the image
        /// </summary>
        /// <param name="xlImage">The image object </param>
        /// <param name="image">The image to set</param>
        public static void SetImage(this ExcelImage xlImage, Image image)
        {
            var ms = new MemoryStream();
            image.Save(ms, image.RawFormat);
            xlImage.SetImage(ms, GetPictureType(image));
        }
        /// <summary>
        /// Sets the font from a font object 
        /// </summary>
        /// <param name="xlFont">The EPPlus Text Font object</param>
        /// <param name="font">The font object.</param>
        public static void SetFromFont(this ExcelTextFont xlFont, Font font)
        {
            xlFont.SetFromFont(font.Name, font.SizeInPoints, font.Bold, font.Italic, font.Underline, font.Strikeout);
        }
        /// <summary>
        /// Sets the font from a font object 
        /// </summary>
        /// <param name="xlFont">The EPPlus Text Font object</param>
        /// <param name="font">The font object.</param>
        public static void SetFromFont(this OfficeOpenXml.Style.ExcelFont xlFont, Font font)
        {
            xlFont.SetFromFont(font.Name, font.SizeInPoints, font.Bold, font.Italic, font.Underline, font.Strikeout);
        }
        private static ePictureType GetPictureType(Image image)
        {
            if (image.RawFormat.Equals(ImageFormat.Jpeg) || image.RawFormat.Equals(ImageFormat.Exif))
            {
                return ePictureType.Jpg;
            }
            else if (image.RawFormat.Equals(ImageFormat.Gif))
            {
                return ePictureType.Gif;
            }
            else if(image.RawFormat.Equals(ImageFormat.Png))
            {
                return ePictureType.Png;
            }
            else if (image.RawFormat.Equals(ImageFormat.Emf))
            {
                return ePictureType.Emf;
            }
            else if (image.RawFormat.Equals(ImageFormat.Wmf))
            {
                return ePictureType.Wmf;
            }
            else if (image.RawFormat.Equals(ImageFormat.Tiff))
            {
                return ePictureType.Tif;
            }
            else if (image.RawFormat.Equals(ImageFormat.Icon))
            {
                return ePictureType.Ico;
            }
            else
            { 
                return ePictureType.Bmp;
            }
        }
    }
}

Remember to add the name space where you want to use the methods, so they are accessible.

using OfficeOpenXml.Drawing.Extensions;

One of our major focus areas for the continued development of EPPlus 6 is improving performance and functionality of the formula calculation. For this reason we have changed a number of classes, interfaces, methods and properties that in previous versions were public to internal as they are likely to go away or change drastically. This means that these items will no longer be accessible via the interfaces of EPPlus 6.

• OfficeOpenXml.FormulaParsing.ExcelDataProvider
• OfficeOpenXml.FormulaParsing.EpplusExcelDataProvider
• OfficeOpenXml.FormulaParsing.EpplusNameValueProvider
• OfficeOpenXml.FormulaParsing.EpplusNameValueProvider
• OfficeOpenXml.FormulaParsing.Excel.Functions.Database.DatabaseFunction
• OfficeOpenXml.FormulaParsing.Excel.Functions.Database.Dsum
• OfficeOpenXml.FormulaParsing.Excel.Functions.Database.Dcount
• OfficeOpenXml.FormulaParsing.Excel.Functions.Database.Dget
• OfficeOpenXml.FormulaParsing.Excel.Functions.Database.Dmin
• OfficeOpenXml.FormulaParsing.Excel.Functions.Database.Dvarp
• OfficeOpenXml.FormulaParsing.Excel.Functions.Database.ExcelDatabase
• OfficeOpenXml.FormulaParsing.Excel.Functions.Database.RowMatcher
• OfficeOpenXml.FormulaParsing.Excel.Functions.Database.ExcelDatabaseCriteria
• OfficeOpenXml.FormulaParsing.Excel.Functions.Math.MultipleRangeCriteriasFunction
• OfficeOpenXml.FormulaParsing.ExcelUtilities.AddressTranslator
• OfficeOpenXml.FormulaParsing.ExcelUtilities.AddressTranslator.RangeCalculationBehaviour
• OfficeOpenXml.FormulaParsing.ExcelUtilities.AddressTranslator.IndexToAddressTranslator
• OfficeOpenXml.FormulaParsing.ExcelUtilities.AddressTranslator.RangeAddressFactory
• OfficeOpenXml.FormulaParsing.ExcelUtilities.AddressTranslator.RangeAddressFactory
• OfficeOpenXml.FormulaParsing.ExcelUtilities.ExpressionEvaluator
• OfficeOpenXml.FormulaParsing.ExpressionGraph.ExcelAddressExpression
• OfficeOpenXml.FormulaParsing.ExpressionGraph.ExpressionFactory
• OfficeOpenXml.FormulaParsing.ExpressionGraph.ExpressionGraphBuilder
• OfficeOpenXml.FormulaParsing.ExpressionGraph.IExpressionGraphBuilder
• OfficeOpenXml.FormulaParsing.ExpressionGraph.GroupExpression
• OfficeOpenXml.FormulaParsing.ExpressionGraph.FunctionExpression
• OfficeOpenXml.FormulaParsing.ExpressionGraph.FunctionArgumentExpression
• OfficeOpenXml.FormulaParsing.LexicalAnalysis.TokenHandler
• OfficeOpenXml.FormulaParsing.LexicalAnalysis.TokenizerContext
• OfficeOpenXml.FormulaParsing.LexicalAnalysis.PostProcessing.TokenizerPostProcessor
• OfficeOpenXml.FormulaParsing.LexicalAnalysis.TokenSeparatorHandlers.TokenSeparatorHandler
• OfficeOpenXml.FormulaParsing.LexicalAnalysis.TokenSeparatorHandlers.MultipleCharSeparatorHandler
• OfficeOpenXml.FormulaParsing.LexicalAnalysis.TokenSeparatorHandlers.SeparatorHandler
• OfficeOpenXml.FormulaParsing.LexicalAnalysis.TokenSeparatorHandlers.SheetnameHandler
• OfficeOpenXml.FormulaParsing.LexicalAnalysis.TokenSeparatorHandlers.StringHandler

• OfficeOpenXml.FormulaParsing.ExcelDataProvider.IRangeInfo moved to OfficeOpenXml.FormulaParsing.IRangeInfo
• OfficeOpenXml.FormulaParsing.ExcelDataProvider.ICellInfo moved to OfficeOpenXml.FormulaParsing.ICellInfo
• OfficeOpenXml.FormulaParsing.ExcelDataProvider.INameInfo moved to OfficeOpenXml.FormulaParsing.INameInfo
• OfficeOpenXml.FormulaParsing.FormulaParser constructors from previous versions made internal. New public constructor FormulaParser(ExcelPackage package)
• Method OfficeOpenXml.FormulaParsing.FormulaParser.Configure() removed/made internal.
• PropertyOfficeOpenXml.FormulaParsing.ParsingConfiguration.GraphBuilder removed/made internal.
• Method OfficeOpenXml.FormulaParsing.ParsingConfiguration.SetGraphBuilder() removed/made internal.
• PropertyOfficeOpenXml.FormulaParsing.ParsingContext.ExcelDataProvider removed/made internal.
• PropertyOfficeOpenXml.FormulaParsing.ParsingContext.RangeAddressFactory removed/made internal.

Static class 'FontSize' has splitted width and heights into two dictionaries. FontSizes are lazy-loaded when needed.

• Misspelled method ExcelNamedRangeCollection.AddFormla has been removed. Please use ExcelNamedRangeCollection.AddFormula
• Method OfficeOpenXml.FormulaParsing.FormulaParser.Configure() removed/made internal.
• Method OfficeOpenXml.FormulaParsing.ParsingConfiguration.SetGraphBuilder() removed/made internal.

Also see Breaking Changes in EPPlus 5