This was more difficult than expected. I ended up calling a pro and it took the two of us to hold the cooktop and move it around. I had to remove the cooktop and tilt it to reach the screws that held the plate that held the knob in place. I don't know who designed this thing, but they did not take into account servicing it.
If I had to do it again, I would also order the gasket that goes around the edge of the cooktop. We managed to salvage it, but I would have liked to replace it.