To remove an over-the-range microwave oven, unplug the appliance, turn off the corresponding circuit breaker, detach the screws holding the microwave to the cabinetry and mounting plate, and pull the microwave out. If the appliance is heavy, have an assistant help with the removal process.
Go to the main electrical panel, and turn off the circuit breaker corresponding to the over-the-range microwave. Ensure that the power is off completely using a non-contact circuit tester, placing it into the receptacle's right slot. If it does not light up, the microwave is safe to remove. Take out all of the appliance's loose parts, including the turntable, and remove the screws holding the microwave to the top cabinetry. An assistant should hold the microwave as the screws are being removed to keep it from falling. Refer to the appliance's user manual to find the location of all screws.
Use a screwdriver to remove the screws below the microwave and against the wall that secure it to the mounting plate. Slowly remove the microwave from the wall with the help of an assistant, and be careful not to let the cord get caught in the access hole of the cabinetry. If installing a new over-the-range microwave, the old mounting plate likely does not need to be switched out as long as the microwaves are the same size.