Not too difficult, I'd recommend a heat gun. Done this recently to replaced a cracked glass lens. Just take cover, 4 metal clips and bulbs out, heat around edges until warm/hot, estimate 80oC as I could still touch it but needed oven glove to work. Get a small philips to start prying and continue with a larger flat head and work your way around glass. Lift off and do your business. To re-attach, re-heat the glue and stick together or use some silicon bath sealant aswell, to make sure water tight.
