How do I change the resolution of ultimate Doom ?
The INI file is not in the ultimate Doom folder
I want to set a 1920x1080 resolution
The picture quality of ultimate Doom is really bad.

Such high resolutions are not possible natively, use a source port like gzdoom or chocolate doom for high resolutions at 16:9.

Sure you can do it. In Galaxy you have to go to "MORE" (next to PLAY button) then "OTHER" and launch "Graphic Mode Setup". In "Basic Settings" mark "FULL SCREEN" and in "Advance Settings" mark "Keep Aspect Ratio". Now you can play with different Scaling Engines here too. Personally I like "hq3x" and "Window Resolution" set to "original". But you can check every single one. Remember after setting to push "Save Settings".