If your security system is a wired system, the battery can be found in the main control panel. There will be a red wire and a black wire connected to the battery. Unplug both wires form the existing battery and plug them back into the new battery (you will need to have purchased a replacement battery). Be sure to connect the red wire to the red terminal and the same for the black wire to the black terminal of the battery. If you have a wireless system, please check your user manual for instructions.