How to convert unwanted heros into medals or other useful components?

You can 'split' heroes which will exchange them for a hero medal and will return a percentage of any invested wisdom medals. To do this go to the hero men and select to view the hero you no longer want.

At the bottom tap the scroll icon to view their skills. Than in the top left will be a small head icon with an X, tap this to split them. If you don't see this icon then the hero is currently stationed in a legion or resource position.