Whilst this page refers directly to the current cloud offering the principles apply for all versions of JIRA. https://confluence.atlassian.com/cloud/add-ons-and-plugins-744721612.html
As previously stated on other questions v3.12 is no longer actively supported and you should be looking to move to a more recent version to remove all the security risks with running such an old version.
Please refer to https://confluence.atlassian.com/jira/backing-up-data-185729581.html for principles of backup and in particular https://confluence.atlassian.com/jira/important-directories-and-files-189891075.html for the different directories and files that should be backed up.
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