If you make a mistake when setting up your hosting package, the next step depends on 1 of 2 things:
I made a mistake when registering a new domain name:
Sadly domain names cannot be changed once they are registered and are non-refundable. This is not because we're horrible, but because the domain overlords have deemed this to be the way since the dawn of domains.
You will have to purchase a new domain name (check the spelling!), and then simply open a support ticket so we can amend your hosting package. This will usually involve just deleting the incorrect package and creating a new one.
My domain is already registered somewhere else and I typed it in wrong when purchasing a hosting package:
Easy-peasy! Just open a support ticket, tell us the correct domain name and we will get that fixed for you in a flash.