It depends exactly what you want to change.
If you signed up for mydomain.x10hosting.com and changed your mind and want say mydomain.exofire.net you can do that from
http://x10hosting.com/account
If you want to go to your site as mydomain.com you would do best to park the "mydomain.com" after changing your name servers if you can or setting the records up.
You can also add mydomain.com as an addon domain so its a little like 2 sites but not