Just as a general thing, editing online isn't a great idea. For one thing, it means that you don't have a local backup of the file, and for another it means that when things do go wrong you lose all of your work. Edit on your machine, save your changes locally, then update or replace the file on the server. You can use FTP ("file transfer protocol"; basically you use a bit of client software that acts something like Windows Explorer or the Mac Finder to copy files from your machine to/from the server; you can find free FTP software online) or the cPanel File Manager to do the updating.