First see if there is one or two small cross head screws holding the unit in (about 4 o'clock and 8 o'clock) if so remove them if you can. The lights should be retained by keyhole and spring screws (3 of IIRC) You should be able to push in the whole headlamp unit and then turn it anticlockwise until the screw hits the end of the keyhole and then the hole is big enough to let the head of the screw through, if that makes sense. If yours isn't doing it you may well need to be robust with the unit, pushing it in and out repeatedly, to get the springs moving and then turning it left and right to free up the screws from the ring. Photos would be better but I don't have access to a camera and headlights at the moment. Good luck.