There is only one way to update. Buy a new server and update that if everything is ok put the new server on line and keep the old one as a backup.
It's a little easier with SMF - if I hadn't gotten lazy about it. Here's how:
(1) put the forum into maintenance mode so the database server is not accessible from outside;
(2) make a copy of the entire file directory (not the database, just the files, like the php scripts);
(3) execute the update/upgrade package;
(4) if it works, great - keep the old copy as a safety backup just in case;
(5) if it doesn't work, no sweat - delete the corrupted file structure, and copy the file directory from step (1) back into it.
I've never had a problem with an SMF-approved update/upgrade not working, so I got lazy and forgot to copy things.
It's basically mostly my fault.