I think I understand and had just asked this in a forum a couple days ago. You cannot change the URL one you've started your hub. I was told the only way around it is to copy the hub, start a new one with the URL you want. Create your new hub and then delete the other.
NOTE: Wait at least 24 hours for the system to recognize the deleted hub as deleted or you could get flagged for duplicate content. Also know that if the first hub has been up...you will lose your Google ranking and it will see the new hub as new, and restart the Google rank.
Hope this helped.