There are two types of windows from which you can fix a game result:
From a Players Window
If you have a Players Window which includes the round information (see Change Layout Dialog Box), double click on the game record for either player. This brings up a Game Result Dialog Box in which you can enter the correct result.
From a Games Window
If the game is still in the current round (that is, you haven't paired the next round), you can simply go to the Games Window (How To: Display Games from this Round). If it is from a previous round, you can create a Games Window for one of the players (How To: Check a Player's Games), or you can use the menu item List Games-Specific Round on the Sections menu.However you do it, double click on the game and correct the result.