JIRA doesn't ship with functionality to revert to older versions and data exports can only be processed by the same or newer versions, so administrators will need to perform a full migrate their content using the remote API. To manually port content across from a newer version of JIRA such as 3.10 to an older version such as JIRA 3.6, you will need to:
- Install both versions. If both are installed to the same machine then you'll need to change the port numbers.
- You can save time if you have any XML site exports created using the old release by importing them back into the old version. Otherwise, you'll need to manually reinstall your plugins and recreate your other settings, such as workflow or permissions, referencing the JIRA upgrade guide for a list of settings that you may need to migrate manually.
- Familiarise yourself with the JIRA remote API services. You'll need to develop your own script to extract all your issues and/or users from the newer version via the remote API and and post them into the older version. I recommend checking out the example XML-RPC scripts for a good starting point.
Though back-porting is not covered by Atlassian technical support, you can obtain assistance with this process by posting to the JIRA development forums. You also have the option of contacting an Atlassian partner to see if they are able to perform this as a service.