UniFi - Upgrading from v2.x to v4.x or later?

There is no direct upgrade path from v2>v4, or later releases. You first need to upgrade from v2>v3 then from v3>v4 (or later). 

You can find the download links for 3.2.10 HERE. It is also still available in the repo, under 'unifi3'. If using the repo method, make sure to comment out the stable line and then uncomment unifi3 line. Revert it after you have migrated.