Retreating is the preferable method for cutting a game short, because its stats will then be added to your profile. If you just leave the webpage or refresh, you will only have a 0-waves games recorded into it. It's also the preferred way of those who can't bear to see their darling soldiers get slaughtered, or for the people who risk their chances of obtaining the Victoria Cross for the convenience of choosing another starting terrain.
The Retreat button is always located at the bottom of the orders menu, even when new orders are introduced into the game, but its hotkey remains 2-6.
WARNING: There is no confirmation for this action!