Take off the rear focus knob, remove the three very small self tappers that hold the circular plate in place, this gives access to the screw focus shaft, which will slide out the back. Take note of where the spacing washer is on the front of the focus screw, it stops the lamp tray getting too close to the front lens. Clean and use graphite grease or powder for lubrication. You may need to take both out to get the lamp tray assembly out. Loosen the cable gland to allow the mains cable, usually thin black teflon coated, to allow the lamp tray assembly to come out fully.
I used to replace burnt P28S lampholders, but found it just as successful to use a dremel with a wire brush to clean the centre lamp contact, then put contactor oil on it.