1. Start the Registry Editor, and find the following key:

   HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\WindowsUpdate\AU

2. If you don't see the WindowsUpdate key, create it. Right-click on the “Windows” key in the sidebar and go to New > Key. Name the key WindowsUpdate, then right-click on that key and create a new one called AU.
3. Click on the AU key and, in the right pane, right-click on the empty space and choose New > DWORD (32-bit) Value. Name the new DWORD NoAutoRebootWithLoggedOnUsers, then give it a value of 1.
4. Reboot.

See Stop Windows from Restarting Your Computer After Updates