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,
https://code.google.com/p/iredmail/w...Yum_Repository
2. if you're happy to go to latest RHEL (5.7 ?), then 'yum update' as root will do it.
NB:
In both cases, first shutdown as much stuff as possible and then do a FULL backup before updating; downgrading is not supported.