First, make sure no small children are around, as this process involves small parts that could be choking hazards. Then: Ensure the light button is in the OFF position. Using a small #000 Phillips-head screwdriver, loosen the side screw. For safety, the screw will not come all the way out. Please read the Instructions for Use for more information. Grip the sides of the battery door and gently tilt it toward the button to release the screw. Then lift straight up to remove the door from the device. Remove the old AAA battery and dispose of it properly. Install a new AAA battery by inserting the positive end in first. Follow the markings in the battery compartment. Immediately replace battery door and tighten the side screw.