Find the 'setting.txt' file in the root of the game directory
My file was located at 'C:\Program Files (x86)\Steam\steamapps\common\Angel at Dusk' but yours may differ. You can always right-click on the game in your Library and click Manage->View Local Files to get there.

Open 'setting.txt' and change it
Open the file called 'setting.txt' in Notepad or another text editor

It should look something like this:

The top part with the "//" are just comments and only indicate what to set below. The setting you want to change is "GRAPHIC_BOARD". I had to change mine from =0 to =1. You may also want to permanently set "WINDOW" to =2 to make it launch in Fullscreen every time and not ask you each time.

I only have two monitors, but I imagine that if you have 3 or more you will just use =2, =3 etc to select between them.

Enjoy the game!
Do it!