Assuming it's the same as the roadster, open boot, access 3 nuts at rear of unit which hold light in place, undo with 10mm deep socket or fingers if not too tight, push out, pull off light connector and reverse to refit. Watch you don't lose springs behind nuts when you remove and don't over tighten when you put it back.
If it's not the same as the roadster I haven't got a clue