Hi.
1) Make a backup (files + database)
2) Copy to your computer -- old css/override.css file and user.functions.php from theme if you do any changes there.
I hope that you remember which files from theme have you edited?
3) Using FTP/SFTP you can replace files/folder directly from latest version of the theme -- folder with theme images you don't have to replace.
4) Now you can copy to server I mean replace those files that you have changed , for example, from (2)
Theme should work again. Soon we will prepare guide for our clients, probably in next week.