Disconnect the microwave from its electrical outlet.
Support the microwave from underneath so it does not fall when you remove the mounting screws. Because of its weight, using a second person to support the microwave is helpful.
Loosen the mounting screws that thread down through the top of the countertop. Turn each of the four screws two or three turns counterclockwise.
Slide the mounting plates that secure beneath the screw heads toward each other. Again, make sure you support the microwave. Otherwise, it will fall at this point.
Pull the microwave down and away from the countertop.