Unplug the microwave. In most cases, the outlet is located inside the cabinet above the microwave.
Request that your helper position his hands under the microwave so that he can hold on to it as you begin to release it from the cabinet.
Check the bottom of the open cabinet to locate the screws that hold the appliance to the underside of the cabinet. Unscrew them by turning them counterclockwise with a screwdriver.
Ask your helper to lift the microwave slightly and then move it forward to release it from the mounting brackets. You and the helper may need to jiggle the microwave a little to work it up and out of the brackets.