If you want to change your cPanel domain name, make sure the domain name you want to change to is not already setup in cPanel as an Addon or Alias domain.
To change the domain name, perform the following steps:
1) Login to the Client Area
2) Click the big Services button, or the Services Menu -> My Services subitem
3) Find your hosting account in the list and click it.
4) Click on the Pencil icon next to your domain name, it will display a text box you can input your new domain name into.
5) Enter the domain name you wish to use as the primary domain name for your cPanel account.
Please note, you must own this domain name already and have the name servers setup properly.
Also make sure the new domain name is not already configured in cPanel as an Addon or Alias domain.
If you get any errors or have any issues, please submit a support ticket.