Pull off the black cap that's on top of the potentiometer. It has quite a snug fit, so some force is needed to pull it off.

No tools should be necessary here. Only your fingers should be sufficient to pull the cap off.

The potentiometer is kept in place by a screw nut. Remove this nut with a 10mm wrench.

Once the nut is off, pull the potentiometer out of the case.

Put the old potentiometer away, but keep the black cap.