Deleting an old save to restart Pokemon X

if the combo doesn't seem to work, you're probably doing it too early. wait for the actual title screen with the legendary on it, not the intro, then hold all three. and move anything you want to keep off the save first (Bank, if you still have it installed) - once it's gone it's gone.

On the title screen hold Up + B + X together. It asks you to confirm deleting the save, say yes and it's gone. Make sure there's nothing on it you want first, it's permanent.