Backing up means taking a copy of a file for safe keeping. One convenient method is to create a new folder named Hide & paste copies of all files you wish to modify into it. In your case new Hide folders could be created in the FS2002\TEXTURE folder & the Panel folder of the particular aircraft you're modifying. Then if anything should go wrong it's simple enough to paste the original files back where they came from. It's good practice to take backups of all files you intend to edit or replace.