Thanks for your reply,
I'm using version 4.5.4_GA_763_UBUNTU6
I'm copying my zimbra folder in Server1 with its permissions (cp -rp) to Server2 (In both server I'm using Ubuntu 6.10 and Zimbra 4.5.4_GA_763) following the steps on
Zimbra's Wiki. But it still saying "UPGRADE FAILED - exiting" after I ran the upgrade process. And the wiki says that it will work if I rerun the script.