Slide the switch on the fob to remove the key.
Once the key has been removed flip over the fob.
With your thumb on the Toyota logo gently press the same switch that you used to remove the key while sliding the back plate up.
Remove the four screws to get access to the battery.
Remove the old battery.
Replace the battery with a fresh CR2032 battery.
Screw all the screws back in.
Put everything back together by lining up the hinges on the fob and the backplate.
Gently slide the backplate all the way down while pressing in the same switch that you used to remove the key.
Everything should be perfectly lined up!
Slide the key back in and your fob should have all the original functionality.