You can change/update the primary domain name of a hosting account located on a dedicated server via your WHM. Once you are logged into the WHM service you should go to the section:

Account Functions -> Modify an Account

Within this section you should select the account that you would like to modify and press the "Modify" button. Once the next page opens, within the Domain field you should apply the new domain name. Afterward, you should press the "Save" button. This way your account will be using the new domain from now on.

Please note: We do not recommend changing the username of your hosting account, since this may lead to inconsistencies with your account and cause issues with it.