You'll be able to cancel a game that has already started by tapping on the white flag box at the bottom left of the screen. Once you do this, you'll be asked if you wish to quit and you'll have to tap on "Resign" to end the game.
Bear in mind that this will negatively affect your statistics because the game will be considered lost. For this reason, if you receive a game request for a game you don't wish to play, we recommend you reject it directly so that your statistics are not affected.