This is how I've done a few updates from FreeBSD 6.2-RELEASE to 7.2-RELEASE. Observe that I do not think this is a recommended or supported way to do it. I've done 2 or 3 updates in this fashion in Multi-User enviroment.
First of all make sure you have the latest utilities needed. I have always updated to the latest current version of the release I have before starting the update sequence - simply because I think all upgrades should be done on a solid ground. If possible make sure you have a solid and working system backup of all essential data and configurations.