If it's just the bulb you are changing then just remove the chrome bezel at the base of the lens from the rubber seal, which will allow you to then pull the lens out . Underneath is the bulb, push in and turn a small amount anticlockwise and then pull out. If its been there for a long time it may well be very corroded and you may have to replace the bulb holder complete but that's easy as well with just a couple of nuts which will probably snap off anyway! Make sure the bulb holder is well earthed.
