Turn off your microwave and unplug the power cable.
Turn the microwave around and remove all screws on the rear panel of the unit. Remove the rear panel to expose the internal components. Notice the cylindrical capacitor. There are two metal terminals on the top of it.
Grip your screwdriver by its insulated handle and place the tip of the screwdriver on one of the capacitor terminals. Slide the tip of the screwdriver to the other terminal and hold it there for a few seconds. This will discharge the capacitor, making it safe for you to continue working on your microwave. You'll know you have succeeded when you hear a pop and/or see a small spark on the terminal.
Locate the circular metal humidity sensor on the board. It is a metal disc-like component with a wire running from it to the board.
Remove the screws holding the humidity sensor in place and unplug the sensor wiring from the board.
Fit a new sensor in the old one's place and replace the screws. Plug in the wiring to the circuit board.
Replace the rear panel and screws.