Saving game
After choosing "Save game" option from menu you'll see this window:

In "Save as" field you should enter the name of your game and press the green button. If there is already game with given name the confirmation window will popup:

If you'll click the green button the existing game will be overwritten, clicking the red button will cancel saving process (game will not be saved!).
You can also save the game by double clicking on the name of already existing one. You will be asked for confirmation as in example above.