Click on Apple icon top left of the primary display
Click “System Preferences” then “Displays”
Once clicked you will see a dialogue window on each monitor detailing the available resolutions, arrangement and orientation options.
Start by clicking on “Arrangement” to correctly align the monitors in relation to one another. The primary one is the one with the white bar across the top. You can change it by simply selecting the white bar and dragging it to the second display. You will immediately notice a temporary red border on the screen indicating it is the primary.
Resolution is usually automatically set to the highest level, but in some cases you might want to go to a lower level for larger text and icon display.
Color calibration is step-by-step process that the OS will walk you through, giving you easy-to-follow instructions for industry accurate contrast, colors and brightness.
You can also change which monitor becomes the primary display – the one with the full-time menu bar – which is found across the very top of the screen.
Notes: Some folks use a second monitor for tradeshow displays or presentations. You can click on “Mirror Displays” in the arrangement menu so the second monitor displays exactly what you see on the primary. Adjust orientation by clicking on the “Rotation” button. For quicker access to these settings, simply click on “Show displays in menu bar” and a display icon will reside in the top right of the primary monitor.