Windows 10 May 2020 Update 20H1 RP build 19041.207 - April 16 Insider

TV2 said:

Can you do an update installation from MCT download on USB thumb drive?
I always thought you could only do a clean install. Maybe I never looked at the other options.

You can perform an in-place upgrade using the USB or the ISO made by the MCT, the trick is that you start it from within your existing Windows. Just plug in the USB (or mount the ISO) then run the Setup.exe you see on the root of the drive to start the upgrade. The method is used in this tutorial.

Repair Install Windows 10 with an In-place Upgrade

TV2 said:

I might try that on the spare PC to see how it goes. It would certainly go faster.
Just nervous I would lose something or everything just by clicking the wrong button!

Until I moved to an unlimited broadband it was my standard method of upgrading all my PCs to the next version - download once, use many.

If anything, it is more reliable than letting windows update do the Feature Update you. It's often recommended as the solution for posters who have had the Feature Update through windows update fail on them, mainly because it gives you the option to NOT check for updates during the install.