Instructions
Level up your Pokemon until it is ready to learn a new move. A message will appear asking if you want to delete an older move to make room for a new move.
Scroll down with the "Down" key on your GameBoy device to the HM attack move you want to get rid of and replace.
Highlight the attack move you want to replace and press the "A" button on your device. Press the "Yes" option when it asks for confirmation.