Seems that you have edited the footer.php of some comercial theme.
Try rename your wp-content/themes/themerenamed folder to use the default wordpress theme for now. Then take a backup of your theme before some changes and try install.
If you just modified the WordPress core, download and replace the existent wp-admin folder.
To avoid some mistakes, you can try rename this wp-admin to wp-admin-old and upload the wp-admin folder complete without changes.
Never change de wordpress core or someting will blow up!
