When it is your turn and you have received a new tile from the Wall, a supplement tile from the Dead Wall (after declaring a Kong or receiving a Flower or a Season tile), or a discarded tile from one of your opponents, you must make your move: either declare out, or discard one of your tiles.
Note: Undo is available only if you have checked the Undo box on the Gameplay page (under User Interface section in the category tree of the Preferences dialog box). Notice that you cant undo anything else than your last discard, and not even that after the tile has disappeared from the discard slot. But if you want to cheat, you can check the Replay box on the same page. This lets you restart the hand and move back and forth in the game by using the history list of the Moves tab. You can also save a game and then reload it.
Tip: The mouse and keyboard operation can be customized. The default configuration for discarding (and undoing) is described above, but you can use alternate methods, if you wish. For more information, see Configuring game controls.