The username for the Control Panel of your hosting account serves as the unique identifier of the hosting account to the system, and it cannot be modified after the hosting account is created.
Note that this username is not displayed anywhere; it is used only for internal reference to your account, and for logging into the Control Panel.
The only possible way to change the username would be to delete the existing account, and then create a new one with the new username from scratch. This can be done upon request. However, the new account would be empty and you would have to upload your content again, losing all settings and data you might have already uploaded. Your mailboxes cannot be preserved either.