There is no changing. If you did, it would only be cosmetic. What you want to do is create a new user account with the proper name you want to use and then delete the one you intended to rename. A few important things to know is that you want to have at least one Administrator account that would be left untouched for troubleshooting/elevated purposes. The account any person would want to use, is a Standard user account (consider this safe practice).
Additional things to consider, before deleting any unwanted user account you want to make sure you backup or transfer over those user account’s user data folder. Once that user account is deleted, that same user account’s user data folder is gone. So beware. I suggest backing up all users folders before adding/removing user accounts.
Now how do we go about doing this?…
Go to you Windows start button and click on Control Panel.
Then under User Accounts and Family Safety, click on Add or remove user accounts.
Then you want to click on Create a new account.
That’s pretty much it.
More reading with detailed explanations and processess: http://answers.microsoft.com/en-us/windows/forum/windows_7-security/how-to-change-user-name-in-windows-7-professional/d95ddaa7-4235-454e-83d6-77232207c7e2