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Where can I find the FAQ on the upgrade process from 2.2.1 to 2.6.4
I wouldn't imagine there is one, as the assumption would be that upgrades are done sequentially.
I would recommend that you at least start by upgrading to 2.6 (although 2.5.6 is still available for download) and read the README carefully, then move to 2.6.4.
See this discussion:
Thanks, will give it try.