O.S Upgrade procedure

Hi Experts,

I am using VCS 6.0 and Red hat 6.1. I have to upgrade Red hat 6.1 to 6.3. Could you please point me to some doucements which have the steps for it.  Can I do this upgradation live,  I mean without halting the VCS cluster and upgrading O.S of one node at a time?

Thanks a lot in advance.