Unplug the microwave's power cord from the wall outlet or shut off the circuit breaker that supplies it with current.
Open the microwave door and inspect the inner screen on the back of the door. You will notice that it snaps onto the main body of the door and there is a space between it and the door frame.
Pry off the inner screen from the rest of the door carefully using a standard screwdriver. Place the screwdriver blade in the crevice around the perimeter of the screen and gently pry out until the screen is detached from the door.
Remove the screws that secure the old handle using a Phillips screwdriver. The number of screws depends on the specific Panasonic model that you have. Remove the handle from the outside of the door.
Position the new handle in the same location as the old one occupied. Secure the screws by hand. Ensure the handle is positioned correctly, then tighten the screws using the Phillips screwdriver. Snap the inner screen back into place and restore power.