Unfortunately you are unable to just change your domain name. When a domain is purchased it is supplied to the relavant database for that domains authority and therefore is unable to be changed simply.
We therefore recommend that when registering a new domain name you check it and then check it again to ensure that it is the right one for you.
If you have a domain name that you no longer use then you can leave it to expire, or you can transfer it to a different owner.
There are currently no exceptions to this with Affordable Hosting so as before we strongly recommend that you check and then double check that the domain name you want is correct before registering it!