Using a small flathead screwdriver, gently break the seal between the white plastic backing and the metal top.

Pry up the corner with the screwdriver

Place the rubber foot back in place.

Squeeze closed the backing and metal (it will snap into place).

Click on trackpad to test.

Insert the battery and you are done.

It may happen again when you move inadvertently a sheet of paper under the trackpad contributing involuntarily to the extraction of one of the rubber feet.