As the guys have said, first rotate the rim and remove leaving the reflector still in the casing. Press the reflector into the casing and twist about 1/4" and it will pop out by spring pressure. Mine if I recall has little arrows at the top showing which way to twist but if none don't worry as the reflector will only rotate in one direction. Refitting is exactly the reverse. Everything is kept together by spring pressure, no clips!

Location: Truro(ish)