If I remember right, the outer ring/trim piece just pops off. Thin blade under it and work it off carefully so as not to break it. Then there should be two screws you have to loosen. I don't think they have to come all the way out, but not sure. I think they just clamp a piece inside the wall so the receptacle and wall box is tight. After you get it loose enough you should be able to tip it to get it out.
Anyway that is the way I remember mine.
Make sure you have the power disconnected.
Let us know how you make out.
2008 Cruiser CF26RK
2013 Ford Eco-Boost
Heavy duty payload - Max tow