Ask someone to help you get your built-in microwave out of the opening. You will need an extra set of hands to pull it out of the opening.
Open the cabinet above the built-in microwave oven and unplug the appliance. If no plug is visible, it is likely directly behind the microwave. In this instance, switch off the breaker on the main electrical panel powering the microwave.
Attempt to power on the microwave to ensure no electric is running to the appliance. Open the door and take out the carousel, if the microwave has one.
Pull off the grill facade or unfasten the screws holding it in place with a screwdriver. Set the grill facade aside.
Shine a flashlight into the recess behind the grill to locate the mounting screws securing the microwave to the wall mounting plate.
Reach into the recess with a screwdriver and unfasten the screws while your assistant holds the microwave up from inside the appliance.
Ask your assistant to continue to hold up the built-in microwave and attempt to slip a piece of plywood under the appliance to keep it from falling behind the small lip on the cabinet below.
Pull the built-in microwave out of the opening and remove the vent, if attached, with a screwdriver. You can now install a new microwave or even hang a cabinet door to make the hole into a storage space.