In the 2 cabinets on either side of the TV, on the bottom front of the cabinet, you will see 2 or 3 slanted holes next to the TV. Ideally, you need 2 people, but if you are careful and strong, one person can do it especially if you use an electric screwdriver.
If you are parked on a slope (front to back) it's best to have the front of the RV facing "down hill".
With one person standing by the doghouse (center console) have the other person just lightly push in the TV at the center top and bottom. If just you, use the non-screwdriver hand LIGHTLY press in the center of the TV. Unscrew and pull out the 2 or 3 screws on each side at the bottom of the cabinet. Once removed, hold (grab) the sides of the TV and pull out and then tilt it forward. The wires and connectors will be visible as will the HDMI connectors. Put the FireTV in an empty HDMI connector.
Lift the TV and tilt upwards. Put 1 screw in BY Hand on each side to "catch" in the original thread holes. Now loosely tighten those 2 screws.
Finally, again, hand start the rest of the screws and then tighten all of the screws. Make sure there's no looseness to the TV now that it's back in place.
You are done.