The knob screws on, but I wasn't able to unscrew the old knob for some reason. I turned the timer to an "off" position, then gently pried the old knob off. After making sure that the metal washer was in place, I then screwed the new knob on, applying gentle pressure. Voila! The knob now turns the timer! This is the 2nd or 3rd time I've had to replace the washing machine knob in the 9 years that I've owned the machine.