Remove the cover by loosening the screw on the top of the sensor. Remove the existing battery.
Press and hold down the sensor’s tamper switch(There may be a spring over the tamper switch. It is not necessary to remove the spring).
Insert the battery into the sensor and wait for the green LED to light.
Release the tamper switch within 4 seconds and press it again. When the sensor is defaulted, the green LED starts to blink.