Hi and welcome to the forum! You can do that by logging into the Client Area, clicking on the hosting account, clicking on “Manage” next to the subdomain you want to delete, scrolling down to “Delete Domain” and clicking it. Also note that, before you do that, if you want your old website to work on your new domain, you must have done these steps first:
If you added it as a parked domain before deleting it, you might only have to change your website URL on the database or on the links you have if you have them hardcoded so it points to your new domain.