A model number sure would have helped in this case. If you want to be 100% sure you need to replace the entire opener. The odds of the person opening your door after you do the following is slim to none but slim is always there. On the back of the motor unit near where all those white wires go you will find a smart/learn button . Hold this in for 6 seconds and it will erase all codes from your receiver and none of your remotes will work. Just to prove it try the remaining remote. Now to program the receiver back to the remaining remote press and hold the button on your remote while you press and immediately release the smart/learn button on the back of the motor unit. If instead of the smart/learn button you find a series of small switches post back.