The lights are screwed on (four or five bolts, can't remember, and also have silicone sealant around the interface with the bodywork.
Pretty-much need to take out the interior trim on that side - 5 mins tops once you've found all the screws. You /may/ need a heat gun (60-70 degrees centigrade recommended I think) to soften the sealant though I got away with using a hairdryer, brute force and ignorance when I did mine a couple of weeks back.
You'll need some silicone sealant for the replacement to make sure it's watertight, though.