Cobra car alarm remote control flat battery replacement procedure.?

I have bought a car with a 'cobra alarm and immobilizer' fitted, with remote locking. The battery has run out on the one remote control, I have no system information on the alarm and wonder if there is advise on how to change the flat battery. Sensible answers please as I don't won't the immobilizer to activate and unable me to use the car. Thank you for your help...Cobra car alarm remote control flat battery replacement procedure.?Guess it has a clip out door to battery.