The above is almost, but not quite correct. It should be

    sudo rm /Library/Preferences/com.apple.SoftwareUpdate.plist

Followed by a reboot.

this will delete whatever server your Mac is looking at to get software updates and will let it default back to the Apple one. I've just done this on an iMac running El Cap which was trying to get updates from an in-house update server that was disconnected years ago.