Hi there,
Domain names cannot actually be changed. They can only be canceled and then another (different) domain name purchased in its place.
Further, domain names can only be refunded within 5 days of when they are purchased or renewed. So since yours was renewed 17 days ago, we would not be able to refund you for the $18. I apologize!
So if you would like to purchase a new domain name, you will need to pay $18 for it, though you can have 2 domains at the same, so the original fee won't go entirely to waste!