Whilst the long plastic nut that secures the light unit to the body is easy enough to undo those side lugs sometimes just don't want to come out of the grommets on the body.....
If this happens I grab a soft plastic kitchen spatula that I keep handy for jobs such as this and carefully insert it at a 45 degree angle where the lense curves around the body and give it a gentle twist.
Not much force is needed and you need to be careful otherwise there is a risk of breaking off the lugs on the light unit.
A plastic spatula means there's no risk of damage to paintwork or light unit, it's worked a treat for me.