See Figure 1. Slowly turn a thin blade in the slot (1) on the side of the fob to separate the two halves.

Holding the fob battery side down, use a small screwdriver or similar tool to press the spring latch (3) away from the battery (2).

Remove the battery and discard.

With the positive side down, press one edge of the new battery (3 V CR2032 or equivalent) against the spring latch. Drop the new battery into the cavity.

Align the two halves of the fob. Snap the halves together.