You want to rename an existing User entry in Active Directory.
This can be done using the Active Directory Users and Computers (ADUC) MMC.
- Open Active Directory Users and Computers
- In the console tree, browse to the folder which holds the user account and Right Click the account and select Rename
- Make the initial name change to the account and hit the Enter key on your keyboard
- Modify appropriate fields in the rename user dialogue and click OK