CentOS 5.6 was released and I updated nearly all of my CentOS machines to the new version. Up to now only one machine made some smaller problems and needs some love to work like it did before the update.
The procedure is quite simple if your are on CentOS 5.5. If you have automatic updates activated and all updates are installed without errors you only need to boot the new kernel by rebooting the machine.
Otherwise install all updates with:
Be sure you have an backup and be sure you know how to use your backup.
If you have some additional drivers installed like VMware does it with there VMware tools reinstall them.