How do I stop those Windows 10 nag messages?
There is a program called GWX.exe that runs all the time (!)
that produces these messages.
Using Task Manager (you can get it with Ctrl-Alt-Del) stop it.
Then so it doesn't start again, rename its folder:
rename GWX GWX.HIDE
You may need to take ownership.
And there is a registry key so a futue update won't replace it: