Note: Remove the plunger gently from the wall before changing the battery.
To remove the blue lid, squeeze, twist, and pull it off.
Locate the battery.
Gently slide out the battery. You may use a flathead screwdriver if this proves difficult.
Note the battery terminals (+, -).
Place a new battery in the device (CR1620).
Replace the lid on the device, making sure that it is seated in all the way.