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Click the Apple icon  on your MacBook. Click System Preferences. Click Displays. Click on the Arrangement tab. Click and hold the white bar at the top of the current monitor icon. Drag the white bar to the other display icon.

Article:
It's in the menu bar in the upper-left corner of the desktop.  This displays the Apple menu as a drop-down. It's in the Apple menu on your Mac. It's below an icon that resembles a monitor in the System Preferences menu. It's the second tab at the top of the Display window. There are two rectangle icons under the "Arrangement" tab in System Preferences.  These represent both displays that are connected to your MacBook.  The one with the white bar overhead is the current primary display. To change your primary display, drag the white bar at the top of one rectangular display icon to the other display icon in the Arrangements tab.  Both monitors will flicker on and off for a second to adjust to the new preferences.  Whichever display is set as the primary display is the default monitor that apps will launch in.