Complete a Puzzle normally on any difficulty. On the "victory" screen, exit the game via the Home button on your 3DS. Upon reloading the game and reselecting the difficulty, the game will prompt you to resume an unfinished puzzle. Select Resume, and the puzzle you just completed will reload with most of the numbers a lighter color. Simply delete and renter any square to recomplete the puzzle. It will have the same time and hints usage as the previous puzzle.