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 email Pokémon support at firstname.lastname@example.org.