If I exchanged a hero by mistake (Pioneer Nora), how can I get her back?

You can't exchange a hero, only the hero medals which you get when you recruit a duplicate hero. If you exchanged a hero medal then you will get wisdom medals and you can't get the hero medal back, but you won't have lost the original hero.

If you 'split' the hero to get a hero medal from it then that hero is now gone and you will have to wait until you recruit it again.