Note: You will need a very small Philips screwdriver (shaped like an x).
The device may or may not have a cover, if there is a cover, unscrew and remove it.
Remove the three rubber tabs on the bottom of the device.
Unscrew the screws located where you removed the tabs.
Remove the cover and locate the battery.
Gently remove the battery from its connections. You may use the screwdriver if this is difficult.
Place a new battery in the device (CR2450).
Replace the cover on the device and line up the screw holes. Press in until you hear a click.
Place the screws in their holes and screw them in.
Press the rubber tabs back into the holes.