Read the release notes and upgrade guidefor the version of JIRA you are upgrading to. The upgrade guide (in particular) contains important information that may be relevant to your JIRA installation.
If you plan to skip a few JIRA versions for your next JIRA upgrade, we strongly recommend that you read the upgrade guides for all major versions between your current version and the version to which you are upgrading. Refer to Important Version-Specific Upgrade Guides for quick links to these guides.
Confirm that your Java platform, operating system, application server, database and hardware still comply with the requirements for JIRA 4.3.x. Newer versions of JIRA may have different requirements and supported platforms to previous JIRA versions. The End of Support Announcements for JIRA page also has important information regarding platform support for future versions of JIRA.
If you have installed any additional JIRA plugins (i.e. not included with JIRA), please verify that they will be compatible with the version of JIRA you are upgrading to. You can find a plugin's compatibility information from the the plugin's home page on the Atlassian Plugin Exchange. Once you have confirmed the availability of compatible versions, you should upgrade your plugins after successfully upgrading JIRA. This can be done via the 'Plugin Repository' in your Administration Console.
Test first!— We strongly recommend performing your upgrade in a test environment first. Do not upgrade your production JIRA server until you are satisfied that your test environment upgrade has been successful.
If you have any problems with your test environment upgrade which you cannot resolve, create an issue at our support site so that we can assist you.
If you have any problems during the upgrade of your production JIRA server, do not allow your users to start using this server. Instead:
Continue to use your old JIRA server — this will help ensure that you do not lose production data.
Also create an issue at our support site so that we can help you resolve the problems with production JIRA upgrade.