how do you concede and start a new game

Near the moves on the right side of the screen, there is a flag titled "resign." Clicking this will concede the game and count as a loss. From there, to start a new game simply go to begin another game as you did with the first one, or more efficiently click "new ..." to play another game of the same time controls. If you played a 30 minute game for example, then "new 30 minutes" pressed will begin another game. This second button is located next to "rematch" button on the pop-up indicating the previous game result.