Deleting the font files and reinstalling the program and components (which took virtually no time at all since it was a reinstall) brought me up to date on the Dorico fonts. Thanks to all for their advice.
Disabling Creative Cloud (and therefore any interference it creates in Windows’ font management) is easy - you just close the application that normally starts up automatically when you log in.
I think Derrek has the right approach - delete the fonts and check they’ve gone as you suggest, Florian, then re-run the Dorico 3.5 installer which will reinstall the fonts and run very quickly in these circumstances. Rebooting before installing any software can help on Windows, even if it’s just avoiding the need to reboot after the installation is complete (to replace one or more files that were open when the installer ran). Closing down Creative Cloud can be done in two or three quick clicks and is perhaps worth doing just in case.
I’m not sure if there is a practical alternative to re-running the Dorico 3.5 installer unless you can manually install the correct fonts by copying them from another Dorico 3.5 installation or go digging through temporary folders to see if you can find them. You can download version 1.37 of Bravura and Bravura Text on GitHub, but the only version of the Petaluma and Petaluma Text fonts on GitHub is the 16 month old version 1.055.
I did my best to deestroy the earlier versions (though it was like whack a mole, though they never died. All I managed was to get to a point where Preview would not work). I then reinstalled Dorico 3.5.
Now I see that the Bravura fonts are v.1.37, while the three Petaluma fonts are:
Petaluma Regular and Petaluma Text Regular: v. 1.062
Petaluma Script Regular: 1.10.
How do I get 1.20 for the Petaluma fonts???
Thanks!
David
I would suggest rebooting, then before you start any applications, delete Petaluma and Petaluma Text from the Windows Fonts folder, then run the Dorico 3.5 installer again. You don’t need to uninstall anything first: Dorico should go ahead and install the fonts anyway.
Dorico 3.5 installs font versions Bravura 1.37, Bravura Text 1.37, Petaluma 1.062, Petaluma Script 1.10, Petaluma Text 1.062 (not 1.12) as reported by david-p.
I did not report that.
I re-did the installation again, exactly as Daniel described just above. The Petaluma fonts disappeared this time. But when I reinstalled Dorico, they were not v.1.20, as Daniel says they should be, but
Petaluma Regular: v. 1.062
Petaluma Script Regular: v. 1.10
Petaluma Text Regular v. 1.062
David
It may well be that I’ve got the version number of Petaluma Text wrong. Petaluma is 1.062, Petaluma Script is 1.10, and Petaluma Text is 1.062 as well, so David’s list of font versions is correct. Sorry for the confusion.
Thanks Daniel! Now we can all relax and get on with mastering all the ins and outs of this wonderful new feature. 
David
Totally agree and Carus (figures) are almost impossible to read, specially naturals. Also the paper size is too small. One of the reasons together with the superior paper quality that Bärenreiter is so much better than all other editions. (at least for Bach). Very often only two or three measures per page in Carus, this seems a bit better in more recent editions. But page size makes a big difference for the available space, also on cost of course.