You can change your domain name by following these steps:
Log into your Control Panel.
Go to the Domain Parking page:
Add the new domain name to the list of parked domain names there.
Allow several minutes for the parking procedure to complete.
If you have purchased the domain name at a third party registrar, you will need to update its nameservers there. The nameservers will be e-mailed to you once the domain name is parked.
Use the set as primary button next to the parked domain name to switch the main and the parked domains. This way, the new domain name will become the main one for your hosting account, and the previous one will become parked.
Changing the main domain name of the account will not affect the email users that are set up. All mailboxes will still work in the same way for both domain names.
If needed, you can remove the first domain name from the Domain Parking page.
Please note that this procedure will not change the username for the account. Once your hosting account is created, the username for the Control Panel cannot be modified.