First, uninstall the Upgrade Assistant.
Open Control Panel > Programs and Features > Windows 10 Update Assistant
Select it then click Uninstall
Open C:\ drive
Select the Windows10Upgrade folder
Then delete it
I've now done this again.
Pernicious update! Maybe it'll just keep coming back every few days.
I have done the same (uninstalling the program and deleting the folder). But a few hours later the Upgrade Assistant is back in the programs.
I am on Windows 10 Pro, but the Assistant even ignores the "Defer Upgrade" setting in advanced options.
I have now diconnected the computer from the internet. So far that is the only solution I know... Well done Microsoft!