How to change a hero's default skills?

[ExalyThor]

Hello! I'd like to change the default secondary skills of some heroes. Some to better match the background (Mephala being a master tactician should have tactics not leadership. If we were to go deeper she should have leadership, tactics and tactics speciality but oh well), some just to make more sense (Torosar has a ballista but the man doesn't know how to use it) and others I can't think about at the moment. How would this be done?