If its anything like mine, you pull out the shaft as if you were going to set the clock, then hold the shaft with either your fingers or some needlenose and the knob should unscrew from the shaft.
Be very careful! The earlier clocks have a screw deep in the center of the knob that requires a small hex wrench. If there's a hole in the center of your knob, then you need to take out the screw first. Hope this helps. Dan