[crackle]

Prise the metal ring apart slightly with a flat screwdriver and work the metal ring off the plastic knob shaft. Once the metal ring is removed the plastic knob should lift off easily.
When refitting smear a little wax from an old capacitor over the control shaft, and in the hole in the knob, refit the metal compression ring to the knob and slide the knob back on the shaft.
You will need to unscrew the ferrite rod brackets before removing the knobs, do one end at a time to help hold the aerial in place.
Mike