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The function source code: Update group function

Disciple_Tools_Groups::update_group( $group_id, $fields, true );

• int group_id (Required): the id of the group you want to update
• array fields (Required): an array of fields like name, phone number corresponding to the new group. See Group-Fields-Format
• bool check_permissions: check if the signed in user has the permission to perform this action. By default this should be true, unless this is called by an automated process.

• The updated group
• WP_ERROR, in case something went wrong.

• url: https://example.com/wp-json/dt/v1/group/[group id]
• type: POST
• data: A JSON string of fields object. See Group-Fields-Format
• contentType: "application/json; charset=UTF-8",
• dataType: "json",

Have a look at the fields page: Group-Fields-Format