Right-click the desktop and open Display settings
Click Identify to see which screen is 1 and which is 2
Click the monitor you want to make primary
Check Make this my main display
Drag the monitor icons to match your physical setup if needed
Click Apply
If you want to swap the numbers, disconnect and reconnect the monitors or change the cable order in the graphics settings
On Windows, open the graphics control panel if needed and set the preferred display order there