How can I switch off Windows Update?

You can’t. Microsoft has changed the way updates are delivered in Windows 10 and the upshot is that they’re downloaded automatically, whether you want them or not. You can, at least, delay the Windows restart that might be needed to a time that suits by scrolling down the Windows Update window and choosing your own restart time.

Unfortunately, this won’t solve the problem of Windows Updates that contain bugs (and there have already been some), but there is a way around it. Just enable System Protection (search for it with the Search box) and Windows 10 will create a Restore Point before each update. That means you can undo any problematic ones, which might be the difference between a usable and unusable PC.