Not tested (yet)!
1. stop zimbra
1a. check if every process is really stopped. don’t want to see stalled mysql or something – happened to me once
2. tar zimbra installation folder
3. tar /var/lib/rpm !!! (I forget often to do this)
4. START ZIMBRA!!! (else upgrade could fail)
5. upgrade
6. if everything works, delete backups, else untar and try again