I think you may not be seeing the screw. You have removed the cover on the bumper, so lay on the floor and look up into the hole and at the top you should see the head of the 'Phillips' screw. Unscrew it, but it does not come fully out, and you will then be able to pull the light unit away from the bumper. To refit, enter the side furthest away from the screw hole into the bumper, then gently press the the light unit into place, hold it in place and screw the screw in. The screw it right up in the top of the hole.