You really don’t want to do that exactly. If you edit the original footer.php code or even the original style.css file then when the Clean Retina theme get’s updated you will lose all of your edits. That would not be good.
Create a child theme instead and consider following these instructions from 4 months ago.
If you’ve edited the theme’s footer.php file them make a backup of those files (you do want to save your changes right?) and just download a fresh copy of that theme.