The light cover is held on by a metal clip, suction and a locating slot.
Remove the metal clip and slightly lift the inner edge ( part closest to the middle of car) then slid/pull the cover to the middle of the car. It is a tight fit but it will come off.
Getting it back on is far harder! place it in the locating slot before you try to fit it fully back on the light.
Make sure the seal in the lid is in place and complete. If you feel it will help smear with a little vaseline or something as the seal will have compressed and creating a good seal to stop the light getting misty with condensation or moisture is important.