Just pull

Get under it with a pair of Dikes and grasp and pry out. Carefull not to cut
it in half. It should pull right out.
It is held in by a series of ribs and you will be able to reuse it.
If you need new ones, grey is a tough color to find but a dealer will have
them, or go to your local auto parts store and look in the HELP section at
door panel fasteners.