Open Accounts preferences in System Preferences. Click on the lock icon and authenticate. Select the user account whose short name you wish to change then press CTRL-mousebutton (or right-click if you have a multi-button mouse) to open the contextual menu. Select Advanced Options from the list. A dialog will open. You will see one line labeled Short Name:. Change the contents of that field to the new name you want. You will see another field labeled Home Directory:. Change it to reflect the new name, i.e., /Users/newname, where newname is the new name you've entered in the Short Name field. Click on the OK button, close preferences, and restart the computer.
