Unlike the home consoles, and the Sony's rival PSP portable gaming system, the Nintendo Dual Screen (DS) does not save game data to the system itself. Instead, game saves are written directly to the game cartridge. This means that when you wish to erase DS game data, you must do so on each cartridge individually. This is usually a fairly simple process, accessible from your game's main options menu. If your game does not have an option to delete data from the main menu, there is a hard-cartridge reset option that works with most titles.
Power on your DS with the game cartridge you wish to delete data from inserted. Once the game loads, check the main menu for an "Options Screen", if your game features this screen, there should be an available option to "Delete All Game Save Data?" Select this option to delete your game data.
If your title did not have the option to delete game data automatically, power off your DS. Remove any game cartridge inserted into the system, then power it up back on. Once you are at the main system menu for the DS, insert a cartridge that you wish to erase data from. You should see an icon appear that corresponds to the inserted game.
Click the icon for your game cartridge, and quickly press and hold the "A," "B," "Y," "X," "R," and "L" buttons simultaneously. This will perform a hard reset of the cartridge data. Note that there are several game titles where this method will not work.
Be sure that there is no game data you wish to retain before attempting a hard reset of your game's data. All data will be lost and cannot be recovered.