Can I replace a damaged firmware of a 3D printer based on Arduino Mega?
It is hard to diagnose the board without hands on experience. It is even more difficult if you get a board that has been "updated/upgraded" by a previous owner leaving it not in working condition.

But, there are at least 2 solutions.


You could buy a new printer controller board, upload new firmware and connect all connectors.
You could try to burn a new bootloader onto your current board and upload a new version of the firmware.