Unfortunately, you may not change the user name associated with your control panel. Once it is set, there are vital components using that name. Some of those settings are used by your web site, and are not controlled by Notable Web.
You may change the user names for any accounts you create, but you cannot change the user name you suggested during sign up.
The only solution Notable Web can offer is to terminate the hosting account, and then recreate it with the desired user name. WARNING: Accepting this solution will erase all web site content, emails, databases, and everything you have done on the server so far.
You may change the user names for any accounts you create, but you cannot change the user name provided by Notable Web; Linux is always root, and Windows is always admin.