First, find the release button on the back to remove the key inside the fob. Press the button while pulling on the metal ring on top of the fob to pull out the key.
Removing the key will leave an opening on top of the fob with two grooves on the inside. Use a flathead screwdriver or credit card to pry apart the plastic grooves. Continue to pop the plastic casing around the fob, being careful not to break the plastic. It should come away in two parts.
Once you’ve popped the plastic casing in half, you should see a black circle of plastic covering the battery. This is to protect the battery from moisture or damage. Pop off the plastic covering and change the battery. Once you’re done, simply snap the plastic parts back together and voila, no more low battery notifications!