You will have to work hard, but as a starting point I would recommend you to reformat and reindent the code, there are out there some tools to do it:

• JavaScript unpacker and beautifier


• JavaScript Beautifier

That as I said, will give you a starting point, you will need to know the code well to rename your variables and functions properly.

The last option would be to consider a rewrite, which if you know exactly what your script is meant to do, can take less time than refactoring the minified source...

And at last but not least I would recommend you to work always with a version control system and do backups often...