You can't. Hidden Powers are determined by the Pokémon's DVs, which are set when you first encounter the Pokémon and can't be changed.
Can you change your Pokes HP move? My Sceptile has HP Poisen and I want him to have HP Fire. Can I change it? If I can - How?
You can't. Hidden Powers are determined by the Pokémon's DVs, which are set when you first encounter the Pokémon and can't be changed.