First backup your entire site, if you can. (I do this twice).
Then just follow the upgrade instructions included in the download carefully. It is very straight forward.
Upgrading is done by 1000's of people so it is not that difficult. A simple Google search on vbulletin.com and vbulletin.org will easily get you additional information if you have a problem. Please backup, and please follow the instructions!
The biggest problem we see around here is the board operator did not have or create a proper backup -- if something should go wrong. Don't be one of them.