Open the cabinet above the microwave, locate the microwave power cable and unplug it.
Remove the mounting bolts that run through the cabinet into the top of the microwave.
Pull the microwave off of the metal rear mount and place the microwave on the counter.
Measure the vent area on the top of the microwave and mark the dimensions on the bottom of the cabinet.
Cut the marks in the bottom of the cabinet with an electric handsaw. This will form the vent.
Replace the microwave on the wall mount and replace the mounting bolts.