Directory and File permissions

I am looking to edit the theme of my wordpress site. I use FileZilla and I followed the WordPress instructions whereby I “Force show hidden files”, and I made the file index.php writable with the numeric value 744. However when I try to edit my Theme in the Themes Editor there is a note stating

“You need to make this file writable before you can save your changes. See the Codex for more information.”

I would appreciate your help with this