Clear out the area around the upper vent. In most built-in microwaves, the upper vent passes through a cabinet so that it can be easily accessed for installation and maintenance. Remove anything stored in this area.
Disconnect the wiring for the built-in microwave to help prevent accidents. Built-in microwaves may be either plugged into a wall socket in the upper vent area, or hard wired into the existing home wiring. If you cannot locate the plug for removal, turn off the power for the microwave at the main circuit breaker. The microwave should no longer turn on when you have cut the right circuit.
Loosen the vent clamps holding the pieces of the vent in place with a screwdriver. Cut vent tape apart with a sharp utility knife.
Grasp the elbow bend in the vent and remove this section by gently pulling it out towards you.
Continue removing other parts of the upper vent that may be in place.