The thread
http://www.zen-cart.com/forum/showthread.php?t=35381 has the required information to do this.
As it says, first try in your local copy. Here is how you could do that.
- Download a copy of site
- backup database
- Install database to local mySql
- Copy downloaded folder to WAMP root
- Upgrade as described in the above thread (replace files, check db changes(might not necessary) )
After all, see whether local copy works. If it is, then simply copy those files to main site as well. Since we have the original site and database, we don't have to worry.