If you are not using Client Management, you can use one of the following Terminal commands to point non-managed client computers to a particular Software Update server. These commands must be run by an admin user.
To change the setting for all users on the workstation:
defaults write /Library/Preferences/com.apple.SoftwareUpdate CatalogURL [URL]
To undo the change and point all users back to the Apple Software Update server:
defaults delete /Library/Preferences/com.apple.SoftwareUpdate CatalogURL
Note: Replace "[URL]" with the URL of the Software Update server, including the port number and the name of catalog file (index.sucatalog). For example, http://su.domain_name.com:8088/index.sucatalog