When the display mode is "Fullscreen (Windowed)", you won't be able to adjust Resolution as it will be automatically set per your monitor resolution.
If you encounter a problem where you cannot change the screen resolution even for "Fullscreen" mode or "Window" mode, then please contact us with the "GameUsersSettings" file.
This is saved in this location as "GameUsersSettings" :
Meanwhile, you can proceed to delete this data and re-starting your game. Please take into account that this will initialize your setting values.