Once an account has been created, it is not possible to change the domain name without deleting and re-creating the account.
For Shared Hosting and Premium Hosting, this can be done by contacting support who can delete the account and re-create it. This will however delete all data including files, mailboxes, databases and settings which will not be restored once the process is complete.
For Reseller Hosting, any sub-accounts would have to be deleted and re-created in the same way from WHM but this can be done without contacting support. The main reseller domain can never be changed though due to restrictions within the structure of reseller accounts.