Usually you will find four wooden "plugs" likely in the side of the facia (trim) and behind them 4 screws Then the wood comes off. Then the TV mount is visible
I knocked the bottom out (carefully) cut the sides so the bottom cut was "Flat" replaced the Bottom and used the same mounting bracket on my wide screen (Same width but not as tall) added a strip of black Gorilla tape to fill in teh remaining gap and job done.
NOTE: You may wish to re-consider the "Smart" Tv. It has microphones and cameras people can hack open.