There are two way to edit that.
1. You can right click on the shortcut of the game > properties > Click on Lauch Options and set your resolution as followed (example here : 1920x1080) :
"......Ra3.exe" -xres 1920 -yres 1080
In this example: I added -xres 1920 -yres 1080
2, You can do follow this video: