Unfortunately you cannot change the username assigned to your free hosting account but, should you really, really want or need to change your FTP login name, POP3 login name and website login name, you can create a new account.

Please note that by doing so, you will not be able to use your existing sub-domain and you will need to recreate your entire site under the new account including importing any databases, modifying any script configuration files and more to successfully change your account.