‘Modify existing game’ should be selected if you want to change an existing game permanently.
-If you choose this option, you will see the existing game description displayed in the box. You can modify this description or erase it by editing the text in the box.

‘Save with a new name’ should be selected if you want to use an existing game to make a similar new game. Your changes will not affect the old game.
-If you choose this option, you must enter a name for your new game to be saved as. You can also enter a description for this game, but this is not required.