Thank you fritzmom, using that link and following all of the directions ALMOST did the trick. I did all the steps (after backing up my iTunes to disk) and got the iTunes 12.6.5.3 installed. It would not start up, saying that the file iTunes Library.itl was made with a newer version of iTunes and so could not start up at all. So I deleted that file from iTunes directory, shut down and restarted. That did the trick, and am now up and running on my Win 10 machine. I will have to go back and rebuild my Audiobooks which got lost in the shuffle, but no problem, all backed up elsewhere
I have now downloaded all my iPhone and iPad "Purchased" app .ipas into this version satisfactorily, with 3 exceptions - all Apple apps which say I have to download to an iOs device, can't be downloaded in Win 10. They are Find My friends, Apple Podcasts and Apple Books, no real loss there.
Also tried out the Updates Tab and it works!!! Finally can keep up again.
Just one further question: How do I transfer the updated .ipas to my Mac iTunes - previous post says to drag and drop to iTunes, but what folder? Mobile Apps is where the old ones are, i think.
Thanks again, and also big RASPBERRY to Apple for making us Mac users have to bend over backward like this to use Apps backup and Update functions that should be freely available to any Mac user who wants it!!