I have a headless PC (18.04) as the home network server and I just had it out for an upgrade and all went well. I installed it back into the cabinet and now realize that I did not set the monitors to be mirrored.
So, now I can see the screen, but cannot do anything get to the menu system. It's a half hour job to get it out of the cabinet in the ceiling.
How can I launch the System Settings "Display" GUI to disable the "inbuilt" and make the external monitor the Main one?
In earlier versions I was able to right-click the screen and select "Change Backgrounds." That dialog had the back-arrow (top left) to take me back to the full list of System Settings and then I could get at the Display.
Is there a way to do this? I just tried launching the Terminal and assume it has launched, but it is in the screen I can't see. :)