To replace an over-the-range microwave, loosen the bolts securing the old microwave, remove the old microwave, insert the new microwave and secure it with the new bolts. Check the dimensions of the new microwave and the bolt hole locations to be sure that they match.
To remove the old microwave, first unplug the power cord. This is often in the cabinet above the microwave. Next, remove the bolts on top of the microwave that hold it in place. This step requires two people: one to undo the bolts and one to hold the microwave. Once the microwave is removed, measure the depth, width and height of the microwave. If there is a mounting place, measure the distance from the bottom of the mounting plate to the bottom of the cabinet, and then remove the mounting plate. If the new microwave has a mounting plate, check the instructions for the proper distance to install the mounting plate below the cabinet. Once the mounting plate is installed and screwed in securely, have another person hook the microwave onto the mounting plate and press it securely against the bottom of the cabinet. Using the bolts included with the new microwave, screw the bolts down through the above cabinet and into the microwave to hold it in place. Ensure that the vent is connected to the old vent outlet, and plug in the microwave.