During a game, you can undo as many moves as you like, all the way back to the beginning of the game, by selecting “Undo” from the “Edit” menu.

“Undo” will do one of three things, depending on what you just did.  (1) If you just jumped one piece over another, “Undo” will back up one jump in the jump chain.  (2) If your last move was a single step, “Undo” will backtrack the single step (which will decrement the number of moves so far).  (3) If the last thing you did was highlight a new piece, “Undo” will simply unhighlight it.

See also: General --> Moves.