Turn off the power to the garage door opener.
Depress the tabs on either side of the lens. Slide the lens off from the garage opener head while holding the tabs in. This action could require a step ladder.
Remove the incandescent bulb from the light fixture socket. Turn the bulb counterclockwise.
Install the fluorescent bulb into the fixture socket. Grip the bulb at its base and turn clockwise until snug.
Replace the lens by sliding it forward until it snaps into place.
Restore power to the garage door.