There are many factors to consider upon upgrading, such as whether JIRA will be upgraded on the same machine, the type of distribution (EAR/WAR or standalone) and etc.
The complete documentation for upgrading can be found on this link: https://confluence.atlassian.com/display/JIRA052/Upgrading+JIRA.
Also, it is recommended to test the upgrade before proceeding with upgrading on the production installation.
Hope this helps.
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