Changing user name

http://http://support.apple.com/kb/HT1428?viewlocale=en_US

tried this, but again, the user name I want to change is not there. I can however sign in as the name I am trying to change (so I know it exists)

Thanks Klaus1. However, when I get here:

In the list of accounts on the left, right-click (or Control-click) on the name of the account you want to modify; choose Advanced Options from the resulting menu.

The name of the account I want to modify is not there. That is my problem.

also. The Mac I am trying to change the name on is the administrator of a network server. Basically, I am the new teacher in a music tech class and want to keep all the old files that were left for me, but change the name to my name. I am currently logging in as the old teacher.

Thanks for any help.