* Learning a new move: This is the most common way to change a Pokémon's moves. Pokémon learn new moves as they level up, or by visiting Move Tutors.
* Using a TM or HM: Technical Machines (TMs) and Hidden Machines (HMs) are items that teach Pokémon specific moves. Once used, they are consumed.
* Breeding: When two Pokémon breed, their offspring can inherit some of their parents' moves.
Unfortunately, there isn't an option to directly delete a move in FireRed. You can only replace it with a new one.