I have the fireplace also and I replaced my tv.There are two screas that come up from the bottom and screw into the bottom of the tv,I removed the grill from the fireplace to get easy access to the screws There is also one screw that is under the middle speaker at the top.You have to remove the middle speaker to get to this screw.To remove the speaker you have to open the two doors above the speaker and remove the door stopper,when the speaker is out you should see the screw for the tv.
Models with speakers in an area above the tv also may have two screws into the TV from the top. To remove them, in the compartment above the speakers are two little knobby screw covers. Remove those and the screws inside. Then pull out the speakers. In the bottom of the speaker openings are the screws into the TV.