This document describes how to change the display settings on a Mac so the local screen is either extended, or mirrored to other connected monitors.
1) Go to the Apple Menu and choose System Preferences, then Displays.
2) Click the Arrangement tab.
3) Check or uncheck the Mirror Displays option. If the Mirror Displays option is unchecked, the display is extended to the right or left of the main display. Note that the blue boxes in the window can be moved by dragging them so they are to the left or right of each other. The position of the blue boxes indicates the arrangement of the available display screens. The Gather Windows button brings the display settings windows from all available screens to the main window so the resolution of each can be adjusted individually, if required.
4) Close the System Preferences window when you are finished.