Instructions
Teach the HM move you want to forget to a different Pokemon. Until you do this, the game will prevent you from making the original Pokemon forget it.
Travel to Lilycove City with the Pokemon you want to forget the HM move in your party.
Travel to the small house immediately to the right of the Lilycove Department Store, the large building found in the north of the city.
Enter the house, and speak with the man inside. Select "Yes" when he asks whether you want him to make one of your Pokemon forget a move. This will open a list of the Pokemon you currently have in your party.
Select the Pokemon you want to forget the HM move from the list. After a short dialogue, a list containing the four moves that Pokemon currently knows will open.
Select the HM move from the list; then select "Yes" when asked for confirmation.