To quickly change how Windows 10 handles multiple displays, press Windows + P. A dark-gray menu titled "Project" will pop up on the right part of your screen. (That's "project" as in projector, not "project" as in a task people work on together.) From here, you can use the mouse or cursor keys and Enter to choose the option you'd like.

The Windows+P key is used to pull up the Project menu from the bottom right side of your screen. So, if you use multiple monitors, you can switch between displays, keep only one of the screens active, duplicate the primary display across all screens, or extend your display area.