Can we exchange orange heros for medal if we have only one. Means no hero medal ?

If you have a hero that you don't want then you can 'split' the hero and receive a hero medal for it and a percentage of any wisdom medals spent on it so far. The hero medal can then be exchanged in the Arena for more wisdom medals.

To split a hero go to the hero menu and tap on the hero. Then select the Scroll icon at the bottom to view the skills. Then use the small icon with an X in the top left by the hero image. (only there if the hero is not currently stationed somewhere) . The hero will be removed.