1. Take out the metal key and split the housing in half by using a flat screw driver.
2. Use a flat screw driver to remove the black plastic that covers the battery.
3. Use the screw driver to eject the existing battery.
4. Insert a new CR2032 battery.
5. Click on the black cover and the housing. Insert the metal key again.
6. Success (no need to resync the key with the car or anything else)