Look for a stable master in New Heim. You will have to pay silver for his services in a popular district. But if you invest resources in the development of your district, you could exchange mounts for free there.
Please note that the exchange of mounts for new ones with a higher rank occurs with a chance! For example, you have no guarantees that exchanging even full stack of four mounts of the 3rd rank will definitely grant you a new mount of the 4th rank.
Please note that the exchange of mounts for new ones with a higher rank occurs with a chance! For example, you have no guarantees that exchanging even full stack of four mounts of the 3rd rank will definitely grant you a new mount of the 4th rank.