Number 43 in the diagram is the switch in question, there are two of them. I would replace them both.
Access is gained by removing the grille from above the door, followed by the screw at the top of the control panel. Once that screw is removed, the control panel will lift up and hinge down, allowing access to the switches. Of course, be sure to unplug the microwave.
The link below is to the part you will need from a highly recommended company rated A+ with the Better Business Bureau. They offer a 100% money back guarantee, EVEN ON INSTALLED ELECTRICAL PARTS, for an entire year! They are the only company to allow returns on installed parts for a refund and offer a FULL ONE YEAR WARRANTY!
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