Changing the hero's starting skills.

In the editor (before starting the game, of course). If the hero has already been placed on the map, open his properties; if not, but you want to hire him in a tavern, open the "Heroes" tab and click Properties, then change it as needed.
By the way, you won't be able to change the hero's specialization (what is displayed in the game under the hero's portrait).
We also have a topic for such questions: All questions about Mapmaking; post your questions there.