How does this mileage reset knob come off
Well that doesn't look stock. If it's similar to other Kawi triples, there should be a screw in the middle, holding the knob on. You might have to remove the speedo to tilt it and see the screw. If you can't get the screw out, since (I think) the knob is not stock, you might consider breaking the knob to get at the screw and then buy a new knob.
This photo shows the parts I bought when I replaced a broken tripmeter knob. Only one screw is needed. I bought two screws to have a spare.