You can undo a bust-out in a number of ways. The standard way is to press the Undo Bust-out hotkey, which is by default assigned to S. The second way is to right-click on a player's name where ever they may be listed on the Tournament page or the Player Rankings page, and select Undo Bust-out. Finally, you can edit a player on the Players tab, and right-click an action in the Tournament History section and undo the action from there.
Notice that the default hotkeys for the "main" actions (bust-out, rebuy, add-on) are all located on the bottom row of the keyboard: X for bust-out, V for rebuy, and B for add-on. The corresponding undo action for each of these is located directly above each key. S for undo bust-out, F for undo rebuy, and G for undo add-on.
