Unfortunately, it is not possible to change your Name.com account username associated with your account at this time. If your username is an email address, you can still update the email address on your account for notification and ownership purposes, but this would not reflect, nor change your username, as this is separate.
You could, however, create a new username and then move your domains to that new account. Here is a link with instructions for transferring a domain from one Name.com account to another.
While it is possible to transfer domains from one Name.com account to another, it is unfortunately not yet possible to transfer products such as Hosting or Email at this time.