You're describing something that's known as Quirky Update. To get it right, you need to follow some rules or the results can't be guaranteed. Check the following article describing it.
http://www.sqlservercentral.com/articles/T-SQL/68467/
Luis C.
General Disclaimer:
Are you seriously taking the advice and code from someone from the internet without testing it? Do you at least understand it? Or can it easily kill your server?
How to post data/code on a forum to get the best help: Option 1 / Option 2