Below is my prepared step-by-step guide on upgrading my Zimbra from version 6 to 7 tomorrow. Let me know if I have missed out any important steps or having un-necessary steps:

0. Schedule a Full backup tonight

1. Download the latest version at: Network Edition Downloads: Enterprise Messaging and Collaboration Software by Zimbra

2. Transfer into tmp folder (for me, it's NFS storage mapped into Zimbra)

3. tar xzvf zcs-NETWORK-7.2.0_GA_2669.UBUNTU8_64.20120410002020.tgz

4. cd zcs-7.2.0_GA_2669.UBUNTU8_64.20120410001942 (stop here today)

5. service zimbra stop (begin upgrade process tomorrow)

6. Take snapshot (I'm running Zimbra on ESXi 4)

7. screen ./ (screen is recommended by Zimbra)

8. /opt/zimbra/libexec/zmdbintegrityreport -r (fix any corruption in the database)

9. sudo zmcertmgr createca -new

10. sudo zmcertmgr deployca

11. zmcertmgr deploycrt self -new

12. su - zimbra

13. zmmailboxdctl stop

14. /opt/zimbra/libexec/scripts/

15. zmmailboxdctl start

16. zmprov fc -a license (to immediately flush the license cache)

17. service zimbra start