An over-the-range microwave oven will need an outlet to run it. This outlet should be installed somewhere in the upper cabinet that the microwave will hang from. This can be an easy installation and addition if you first locate the outlet below that feeds the range. Generally there is a 120-volt outlet behind the range.
Measure how far off to one side it is and then mark the same distance inside the back of the cabinet. Mark the outlet box hole and use a jigsaw or sawzall to cut the opening, but check to make sure the outlet below is turned off at the breaker. Take care not to cut too deeply into the wall because the elecrical wire may be back there.
Once the hole is cut, you may find the feeder wire running to the outlet below. If so, remove the outlet below and remove the cable clamp in the electrical box. Now, connect a piece of 12-2 non-metallic sheathed cable with a ground and pull the wire from the new hole you cut until you get the new piece you are pulling into the cabinet hole. Install both in the new box and insert in the hole. Pigtail the wires and install the new outlet and coverplate in the cabinet. Install the lower outlet and its cover and the installation is complete. Turn the power back on at the breaker and check both outlet for power with a voltage tester.