Make sure it is off and use one of two methods:
(1) a potato with a small section of the end cut off to give a flat area up against the bulb base turning counter clockwise or to the left will loosen the base enough to get it out. If not
(2)
again, to make absolutely sure no power is flowing, unplug the microwave at this point. Use needle nose pliers to get the metal on the base of the bulb (I assume the glass section came off without the base which happened to me) and turn it so it is turning to the left to loosen. This should get is far enough to get out and replaced.
Coach Cupp, December 2017