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 SoxDomains.
You may change the user names for any accounts you create by going to WHM and clicking the Modify Account link. However, you cannot change the user name you picked during sign up.
There must be a way to change my signup username.
The only solution SoxDomains 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.