Firmware V03 for Windows
How to update W012 GameCube Adapter for Wii U?
Download the firmware from https://www.mayflash.com/Support and install it on your Windows PC.
Push the switch on the side of the adapter to “PC mode” and plug the black USB port to your PC.
Select “Update Firmware”.
Donot disconnect the device until you are informed that the updated was completed successfully.