@wonko I ran this in Ubuntu : xrandr |awk '/connected/{print $1,$2}'
And verified that it is HDMI-0
and it worked! I have Recalbox running on the external screen - thank you for the help!

@wonko I ran this in Ubuntu : xrandr |awk '/connected/{print $1,$2}'
And verified that it is HDMI-0
and it worked! I have Recalbox running on the external screen - thank you for the help!

@wonko I was able to edit the .conf file, this line:
;system.externalscreen.prefered=

I deleted the ;
I tried these values after the equal sign:
hdmi-0
hdmi-1
hdmi-2
hdmi0
hdmi1
hdmi2
No luck! I guess I will look at some more of those tutorials.

Thank you for the links. Part of the problem is, whenever I try these solutions, I am trying them on a broken screen and I can only see the left side. I decided to try booting from usb into a Ubuntu live environment and edit the .conf file from there.

I read this post because my laptop screen is broken. My recalbox instance is installed on the laptop hard drive.
Editing the .conf file sounds easy enough but I don't know how to get there.
I can see the MAIN menu well enough to go through those options but I don't see a way to get to the file system. I don't see any display options to change under 'UI Settings'. Any suggestions as o how I can use an external monitor connected via HDMI?
THanks!