You don't. The main school of focus that you chose when you created that character (Balance) cannot change.
If you wish to be a Fire Wizard, you need to start a new character (don't worry you will keep your Balance character and can have up to 6 Wizards at a time).
Click the "NEW" button on the classroom screen to start a new Wizard, and this time, if you want to be fire, choose to "Skip the Quiz" and just select "Fire".
you cant change your wizard only make a new one. but for crowns you can buy back your training points and redo your spells you get. so if your deck is not working out for you then you can get different spells.
