Instructions
Insert the "Pokemon Diamond" game cartridge into your Nintendo DS.
Turn the Nintendo DS system on by pressing the "Power" button and select the "Pokemon Diamond" icon to start the game.
Press "Start" or "A" at the title screen to go to the main menu.
Select your save file and hold up-left on the directional pad, the "B" button and the "Select" button simultaneously. You will be asked if you wish to erase your game profile. Select "Yes" to erase the data. You can now start a new profile and save your game.