Turn off the system by entering the deactivation code into the control panel. Never disconnect the system when the power is on.
Unplug the alarm siren and remove the backup battery if it has one.
Unplug the control panel and remove the backup battery from it as well. Remove the siren and control panel from the wall and pull out the power cables.
Peel off each of the sensors from the doors and windows. Each sensor has two components: the transmitter and the magnet. Pull off both, because the transmitter will be on the door or window frame while the magnet is on the door or window itself.