There is a hole at the end of the round part of the handle. It is threaded to take a screw, but I forget the size. You run a screw into the hole to force the handle off
It’s a 4-40 screw. To loosen or remove an outer handle turn the 4-40 screw in finger tight until it stops. Then using a screwdriver tighten a hair more until the handle loosens up. Then remove the screw.
Its shown on my website.
The inner B or C handles are a bit harder to remove or loosen up and also tough to get back together. I normally remove everything inside the case including the transformer core and inner handles when restoring a zw. You could try removing just the 2 screws and remove the whistle switch, 2 layers of insulation plus the electrical contact. Clean the shaft and reapply grease to the places where the inner handle shaft touches or rotates on the Bakelite case. Apply some conductive grease on the center of the shaft where the electrical contact touches. Then the fun part of reassembling the electrical contact, 2 layers of insulation the whistle switch then the two screws. Some of the steps are shown on my diode replacement page.