Niantic has put together the following guide in regard to Fast Attacks and Charged Attacks in Pokémon GO:
Fast Attacks and Charged Attacks
By default, all Pokémon have two attacking moves: a Fast Attack and a Charged Attack. Fast Attacks deal a small amount of damage, while also powering up a Charged Attack. Charged Attacks can be used once they’re fully powered up and they also deal larger amounts of damage.
You can teach your Pokémon another Charged Attack by exchanging Stardust and Candy. Once learned, it will also be available to use in Raids, Gyms, and Trainer Battles. This option is accessible in the Pokémon summary page.
Using TMs
Additionally, you can use Technical Machines (TMs) to make a Pokémon forget an attack and learn a different one in its place.
Fast TM
Teaches the Pokémon a new Fast Attack
Charged TM
Teaches the Pokémon a new Charged Attack
Source: Official Pokémon GO support
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