Is isn't , it's just that on some games such as dead island they don't want people hacking it or getting trophies with out doing them . So they put a lock on there saves
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All game data is saved on the game cartridge itself. The DS does not have internal memory for saving game data - the DSi has a memory card, but also does not save game data to it.
First you start a new game and then just save over your old saved game.
If you mean a game save [under Game > Saved Data Uility on your XMB] then the games that you play will automatically load those for you. If you mean a saved homebrew game, read the readme.txt they usually come with, it will tell you to move all the data to "psp/game" or maybe "psp/game150" ect..
Manu-load game-dele save game
you have to delete the hole game its self and then get it back on your d.s but you also have to delete the saved data.