To submit a file go to the "Create new file" button and after pressing it you will be presented with a dialog box with the text "Name your file...". In here, insert the name of your file and below this copy the text of the file in to the main dialog box. Then, below the main box you'll see the dialog box titled "Commit new file" where you should put a summary of what the file is about, then press the "Commit new file" button to complete the commit.

To edit an existing file, click on the name of the file to view it and then go to the far right and click on the pencil icon to enter the editing view. Then change the file as desired and write a commit message below, then press the "Commit changes" button to complete the commit.