You need to push the circlip that holds the handle onto the spline. Basically get a nice fat screwdriver, and push it towards the centre of the winder, the handle of the screw driver being at the top/end of the winder handle.
A little push should move the circlip, and you can then easily pull the handle off.
To refit, put the circlip back into position and simply push the handle back on.