Keith - both unscrew from the static centre column (the three grub screws hold this in place). But it seems to me it is best to undo the bottom collar first. The top, the 'main' gear knob, is then loose and may be unscrewed very easily. Just in case I needed it, I marked the centre collar so that I could replace it in exactly the same orientation so when the top was replaced and screwed down fully it becomes tight with the shift pattern insert orientated correctly. Bottom collar then tightened up to it.
