Welcome to the forums,
You can find directions at the bottom of the release notes via:
4.5.11 & 5.0.2 Released!
Network Edition or Open-Source Edition?
http://www.zimbra.com/forums/announc...html#post62754 so we know if you're NE or FOSS later on, as often commands/options given differ.
For instance NE has:
CLI - zmbackup Network Edition Only - Zimbra :: Wiki
More:
Backup and Restore Articles - Zimbra :: Wiki
Step 1: Backup
Step 2: Backup your backup

Steps 3+ from downloading the packages to the order of machines to run the upgrade on - can all be found in the release notes.
You can do things like running the DB integrity check (/opt/zimbra/libexec/zmdbintegrityreport) ahead of time, etc. Can you give us a HW description of the macines (& your backup options), store sizes, & user count anyways?
Don't forget to give yourself extra time just in-case you have to restore to a backup.