Can't update firmwarre, board has been retired

Hello, I have an old version of firmware on my quadchopter’s APM, the board is from generation 2.5, I tried to update the firmware through mission planner, initial setup and update firmware, but during the installation, I’ve got a message that says: “The board has been retired, Mission Planner this will upload the last available version to your board (AC 3.2.1/AP 3.4.0)”

Does somebody know why is that and how to update the firmware to above 3.3 AC?
Thanks from advance

That board was retired some 2 or 3 years ago. Not enough memory and speed to run anything past 3.2./3.4.0. It works well on 3.2- I still fly one, but you will not be able to go to anything past that version.
There is ongoing work using some of the low cost “racing” quad boards that do work on the latest software should you wish to upgrade your FC and not spend as much as you would on a Pixhawk or Pixracer. Check these boards before you buy something as there are a few “low-cost” “Pixhawks” out there that are nothing but a problem. Many are OK though, just certain venders to best be wary of.
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I am also facing same issue. And when I upload 3.2.1 it is arming escs. Please provide solution.