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
That's right, but incomplete.
Once, I deleted the folder C:\windows10upgrade and thought that was it. But some day after Windows 10 just installed the Windows 10 Update Assistant ("the Assistant" after this) again.
Then I set up a rule monitoring program writing files to C:\windows10upgrade, and it let's me trace down to another folder: C:\Windows\UpdateAssistant.
I cannot think of a way to block this process because the command which tried to create this folder is:
C:\windows\system32\msiexec.exe /V
Therefore I installed a HIPS product, and set another rule which stop from writing to these two folders.

This is a Chinese HIPS product called 火绒博锐, sorry that it doesn't have multi-language support. I believe most HIPS software have this type of customized rules.
And I don't know how to do so just by Windows itself. I've tried to set folder ACL to "Everyone-Deny-Any type of access", doesn't work. Maybe just because it's done by the system itself?