If you are simply trying to change the Registered to: information which appears in the System applet, you can do so by modifying the following information using Regedit.
To open the registry editor:
Start >> Run >> type regedit >> press [Enter]
With the Registry Editor open, you want to navigate to the following registry key:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\Current Version
The two entries you will want to modify are:
Simply right-click the entries and choose Modify. Type the desired information.
If you are trying to rename the user account and associated profile information, this is a little more involved refer to the following post:http://www.bleepingcomputer.com/forums/t/136197/change-the-account-names-in-documents-settings/
Please read the links that are included in the post BEFORE you begin changing things to prevent corruption or data loss.