Click on the Window that is partially off the desktop so it is the active Window.
Option 1
Use the Windows Key plus one of the arrow keys to move/resize a window – see below.


Reference: https://support.microsoft.com/en-us/windows/experience/snap-your-windows
Option 2
If you can access the Title Bar. Left click on the Title Bar at the top of the Window and drag the Window to reposition
