Open the cabinet over the appliance to unplug the microwave power cord from the outlet inside.
Take out the bolts in the bottom of the cabinet where the hardware attaches to the top of the microwave, using a screwdriver. Have your helper support the microwave while you extract the bolts.
Push up on the bottom front edge of the microwave, and tilt it at an angle to unlock the tabs on the back from the metal support bracket bolted into the wall.
Lower the appliance to the oven surface with help from your assistant.
Pull the power cord gently from the cabinet opening to release the electrical plug. You can now take the microwave to a repair shop or replace it with a new model. Depending on the brand and size of a new model, the support bracket on the wall might also need to be replaced.