Open up the old over-stove microwave oven. Remove anything inside, such as glass carousels, temperature probes or other equipment.
Open up the cabinet above the microwave oven. Remove anything stored in the cabinet.
Unplug the microwave oven from the socket.
Remove the vent running from the top of the microwave through the cabinet. This may be held in place with a series of clamps or tape. Loosen the clamps with a screwdriver or cut away the tape with the utility knife before removing the vent pipes.
Have an assistant support the microwave. Loosen the screws in the bottom of the cabinet holding the microwave in place.
Look on the sides of the microwave for any trim panels and remove these, if present. Some microwaves use these to cover up the support brackets along the sides. Slowly tilt the front of the microwave down. Support the oven at the back and slowly pull it away from the wall. Unclip the microwave from any support brackets, if needed, as you lower it.
Remove the mounting plate from the back wall.