The game is automatically saved every 10 minutes, as well as every time you change player slots or quit the game. In order to use savegames to change the outcome of the game, you need to back up the savegame files before you make a choice that you might want to "undo" later.

For example, in a situation where you are getting ready to move to the next generation, and you think that you might want to change your mind about which child to select, you should back up your savegame files using the procedures in this post before you click the New Generation button. Then, should you later decide that you made the wrong choice or want to see what would happen if you did things a little differently, you can restore the backup and try again.

In the case of an aging couple that has had no children, backing up the game early in the marriage (paused, of course) can allow you to 'replay' that generation should they die without heirs. Even if you back up the game very late in the generation, you can use the backup to continue to 'revive' them and try to get one of the orphan events. It's technically manipulating the game's outcome, but it does offer a way to avoid having to start over after many generations of game play.