This plan covers:
First, navigate to your system's display settings by either right-clicking on your desktop and selecting Display Settings or by typing Display Settings into the Windows Search feature located on the taskbar and pressing Enter. From here, click on the Identify button and a number will appear on each connected monitor. These numbers directly correspond to the numbered squares in your display settings, click on the corresponding numbered square you wish to become your primary monitor. Finally, click on the Make this my main display checkbox underneath the Multiple Monitors drop-down menu. If this option is greyed out, then the monitor you have selected is already your main display.
To change your primary display on MacOS, open System Settings and navigate to Displays. Next, click on the Arrangment tab to view your current display orientation. One of these displays will have a thin white menu bar at the top, click and drag this menu bar to the display you wish to make your primary display. The new primary display will now be indicated by a red outline in the Arrangement tab.