Find the main control box. It's beige-colored metal, usually in a closet, basement or utility room.
Enter the "installer code" into the control panel to disable the alarm system. This code should have been left by the technician who installed the system. If you do not have this code, the alarm will go off while your are replacing the battery.
Unplug the control box's power cord or cut power at the circuit breaker.
Open the front panel of the control box.
Remove the red and black wires from the battery inside the control box. Replace the battery. Reattach the wires.
Replace the cover on the control box. Reconnect the power.
Reinitialize the system at the control pad.