TicTacToe game needs optimizing
You could perhaps look at separating the win condition by sticking it in another method then call it in ActionPerformed.

Outside of that, you could replace:

if(count % 2 != 0){
        letter = "X";
    } else {
        letter = "O";
    }


with a ternary operator such as:

letter =(count%2!=0)?"X":"O";


That's all I can think off the top of my head.