If there's a piece of wood trim along the top edge, look to see if there are 3 mounting screws. If so, remove the screws and the trim. The TV will be hanging on 2 horizontal bars. The bottom will have a screw locking the TV bracket against the bar. Use a mirror and flashlight to locate the screw (or maybe 2) and loosen. Then you can pivot the bottom of the TV toward you a little and lift up off the top bar.
Because there are cables and cords attached, it's handy to have a 2nd person helping with them while you lift.