Microsoft Removes the 'X' From Windows 10 Update Leaving No Way Out (theregister.co.uk)
simpz writes: The Register reports that Microsoft has changed the Windows 10 update dialog and no longer shows the "X" close button. They say once agreed to there is no obvious back-out method and it is now out of step with Microsoft's own documentation on this. They have a screenshot of this. As noted above, the latest move is out of step with Microsoft's Knowledge Base documentation, which says you can re-schedule your upgrade.
Try Alt-f4. Works on many apps and usually forgotten by most devs.
https://www.grc.com/never10.htm