If you’re using a Picade PCB (not HAT) then this guide might help: New 2.x Configurable Keyboard/Gamepad Firmware
If you’re using the HAT then you need to tweak the keybindings in /usr/bin/picadehatd, see: https://github.com/pimoroni/picade-hat/blob/master/daemon/usr/bin/picadehatd#L63-L78