How can I override an already saved Diamond and Pearl Pokemon game?

Answer 1

you click on new game on the main screen get through the intro stuff and when it possible for you to save you save and then it will say warning there is a saved file do you want to save over it, hit yes and then it saves the new game.

Answer 2

You first have to delete the save by pressing Up, Select and B at the same time. It will ask you if you want to delete your save. Select Yes, and then start a new game and save on that.

Answer 3

At the screen with ho-oh u have to press up+select+b to overwrite the file