Volkswagen Vento

both of dad n mum car use the same alarm system (same remote control)
i found 1 spare alarm remote control and tested for both of the car but is doesn't work
due to the serial no. writen on the circuit, it is the same for 1 of the car remote control of the bot car so i wana try to adjust so that it can be use for 1 of the car
can u teach me how 2 do?
(bettery had been changed into new battery but still cant be use)