Exterior wall repair

I am remodeling a trailer and have come up on a problem and I'm not sure the best way to repair it. The board that runs along the bottom of the wall of an exterior wall under a window is rotten. The leak has been repaired. Do I make a dummy wall of sorts so I can pull the bad wood out to replace it? I have the paneling and insulation out already. The studs are good and so are the floor joists. I will be replacing the subfloor. Here is a picture of the wall