Picade Board Firmware Update

Hi, I’m Paul, new to this forum and also a total Raspberry Pi fan.

Recently purchased the PiCade from the pirates… Awesome bit of kit. got it all working with RetroPie.
but… Mame requires coin button press of ‘5’, which is not mapped through the PiCade PCB.

I’m a total newb when it comes to boards, wires, etc… I’m a software person.

How does one compile a new sketch to be flashed to the Picade PCB? All I want to do is change the coin button to be ‘5’ instead of ‘c’

Your help is most appreciated.

P.