The admin account is hidden by default. If your user account has full admin rights you don't really need to use it. But if you want the option:
If the Welcome Screen is enabled (You have to log on to a user name) press Ctrl-Alt Del twice. You will get the Logon to Windows dialog. Enter Administrator, and then the administrator password.
If you want to add the administrator option to the user names permanently open a command prompt and type
This will open local users and groups. Double click the Administartors group and remove all accounts EXCEPT the Administrator.
Open either Users or Power Users groups and add back each of the users you removed.
You cannot change the administrator name.