Pokémon GO does not store account passwords. Your account password is maintained by your login provider: either Google or Pokemon Trainer Club. To change or reset your password, please visit your login provider website.
Pokemon Trainer Club: visit the Pokemon Trainer Club website to reset or change your Pokémon Trainer Club password. For further assistance with Pokémon Trainer Club, you can visit the Pokémon Support Help Center.
Facebook: visit Facebook’s Help Center for more information on how to reset or change your Facebook password.