The users in Windows 10 officially are unable to refuse from downloading and installing system updates because such option disappeared. But there are two simple ways to disable automatic updates in Windows 10.
Switch to metered connection to prevent updates downloading
Stop wuauserv service to completely turn off Windows update
Press Enter or OK:
Double-click the Windows update service.
In Startup type select Disabled.
Stop the service.
This is how it should look like: