This is a real issue for some users of specific projection software such as OpenLP, Easyworship, VideoPsalm etc., especially when using one monitor for desktop work and the second monitor is hooked to a projector.
The answer is to delete some registry keys and then restart the PC with only one monitor - the one you want to use as main monitor - monitor 1. All other video leads MUST be removed FROM THE PC. This will then force the OS to assign that monitor on that video output as 1. Then you can attach the second monitor, detect monitors and away you go. Do the usual dragging to how you like to use them if you wish. Make sure you've then assigned the monitor you want as the primary monitor and (probably) extended the displays.
The registry keys you need to delete are these:
HKEY_LOCAL_MACHINE\Systems\CurrentControlSet\Control\GraphicsDrivers\Configuration
HKEY_LOCAL_MACHINE\Systems\CurrentControlSet\Control\GraphicsDrivers\Connectivity
Don't forget to make a backup of your registry first!
Many thanks to https://support.displaylink.com/knowledgebase/articles/736116-windows-display-properties-identify-numbers-are for this. Took some tracking down. But it works.