Changing on any domain including the primary domain name of VPS is made pretty easy via WHM.
- Login to the WHM (The login information is in your welcome email).
- Select "Modify an Account" on the left menu under "Account Functions".
- Select the domain you wish to edit and click Modify button in the bottom of the page.
- Replace the primary domain name from the textbox called "Domain" with the domain name that you want change.
- Confirm changes by pressing on save button.
Congratulations, now you have a new one primary domain name associated to your VPS.