There is the rubber knob cover and then the "spacer". Take a small flathead screwdriver and pry up the rubber cover just under the cover, leaving the spacer intact. The new cover pushes down into the spacer, then you have to use the screwdriver and go around the edge of the cover to force all the way into the spacer. It's not easy, good luck.
If you need to replace the plastic part of the entire knob assembly, forget it.