Instructions
Place the Nintendo DS on a flat surface and set the Pokéwalker directly in front of it. Make sure the infrared portion of the Pokéwalker is facing the infrared port on the DS. Line them up to prevent any connection errors.
Turn the DS on and select the "Pokémon " icon. Press "Start" at the title screen and select "Pokéwalker" from the options list.
Press the "X" and "L" buttons at the same time, and then press down on the D-pad. The game will ask if you are sure that you want to erase the Pokéwalker data. Select "Yes." You can now place a different Pokémon onto the Pokéwalker from your new game.