Assuming you have a RH subscription, there are 2 options
1. If you want to only go to 5.5, you can use this guide,
2. if you're happy to go to latest RHEL (5.7 ?), then 'yum update' as root will do it.
In both cases, first shutdown as much stuff as possible and then do a FULL backup before updating; downgrading is not supported.