How do I reinstall Apple Software Update in Windows?
Ah, Software Update can be reinstalled by running AppleSoftwareUpdate.msi in the Boot Camp Windows support files under Drivers Apple

(To get the Boot Camp Windows support files, boot into Mac OS and use Boot Camp Assistant to save them to e.g. a USB drive that Windows will be able to read -- see https://apple.stackexchange.com/a/247252/5231)