Unplug the opener. Stand on a stepladder beneath the main motor unit of the garage door opener and remove the screws holding the plastic lens over the light bulb in place. Some models have two tabs which, when depressed, release the lens.
Hold a cloth in your hand and unscrew the pair of light bulbs from their socket. The cloth protects your hand in case the bulbs are still warm from use.
Screw the cover back into position. The unit won't light up without its bulbs, but you may still hear a click as the unit tries to activate them.