Battery Tutorial: Door Sensor (DW20)
"You will need one CR2 battery and a small flat head screwdriver to change the battery in your door sensor.
Before changing the battery, make sure that your system is disarmed and note that you may here some beeping from the panel when you open the sensor.
Go to your door sensor and use the screwdriver to pop the sensor cap out of the door.
Slide the circuit board of the sensor out and then remove the old battery and replace it with the new CR2 battery.
Slide the sensor back into the door frame and replace the cap.
Remember to safely dispose of the old battery.
For more information and tutorials, please visit support.vivint.com."