Setting the software updates server from the command line:

sudo defaults write /Library/Preferences/com.apple.SoftwareUpdate CatalogURL "http://example.local:8088/index.sucatalog"

Shows the software updates settings:

defaults read /Library/Preferences/com.apple.SoftwareUpdate

Resets the catalog URL to point to Apple:

sudo defaults delete /Library/Preferences/com.apple.SoftwareUpdate CatalogURL