Click the Apple logo in the top-left corner of the screen. A drop-down menu will appear. It's near the top of the Apple drop-down menu. You'll find this computer monitor-shaped icon in the upper-left side of the System Preferences window. It's at the top of the page. If you want to extend your desktop using the second monitor, make sure that the "Mirror Displays" checkbox is unchecked. If you want to display the same content on both monitors, you can leave the "Mirror Displays" box checked. If you want to use the second monitor as your main display, you can click and drag the white rectangle at the top of one of the blue displays onto the second display.

Summary:
Open the Apple menu . Click System Preferences…. Click Displays. Click the Arrangement tab. Uncheck the "Mirror Displays" box. Change your default display.