The ghost.io subdomain (e.g. yoursite.ghost.io) can be changed within Ghost Admin, in the Ghost(Pro) > Domain settings area.
To change the subdomain, click Edit next to the current subdomain, and enter the new subdomain.
Once updated, the site address will use the new subdomain going forward.
Note: If you have an active custom domain attached to the publication, it will need to be removed first, before changing the subdomain. The custom domain can be reactivated once the DNS records have been updated to match the new subdomain path.