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We also encourage you to explore all things Zimbra with our team and members of the community.
Full details on how to upgrade and the versions that can be upgraded can always be found in the Product Release Notes which ara available with every Zimbra Release, you should always read them in detail and make a backup before you upgrade.
BTW, you could have found this answer by searching the forums first.
Bugzilla
Wiki
Source
steps for upgradation 
Linear Mode
