First you need to remove the "bucket" which holds the CO2 cartridge. Then if possible, clamp the gun in a vice (protected by rags) so that it isn't going anywhere. Then use the 3/8 allen wrench and insert it into the now exposed face of the cartridge changer assembly (you will see the pierce pin... little silver "pin" surround by a flat, white seal). Turn the allen wrench like you are unscrewing a large screw. Hopefully with a little work, the rest of the assembly will come out of the vertical air adapter (the portion attached to the front of the trigger guard with the hose running to it).
If the vertical adapter moves when you start cranking on the cartridge chamber... STOP! Make sure the two screws holding the adapter on to the guard are tight. If they are tight and the adapter still moves, remove the screws and take the adapter off the gun and put it in a vice (protected with rags) and then try to remove the changer assembly.