Documentation for JIRA 5.2. Documentation for other versions of JIRA is available too. 
Upgrading from JIRA 3.13.x to 3.13.4
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Upgrading from JIRA 3.12.x and earlier
In addition to the above, please read the Upgrade Guide for every version you are skipping during the upgrade. The complete list of Upgrade Guides is available here.
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