Launch the game, set the resolution and the video settings according to your preferences, then minimise the game (do not close it) and go to the installation path (ex: D:\Games\Steam Library\SteamApps\common\Counter-Strike Global Offensive). Now go to the "csgo" folder, then the "cfg" folder, and there search for the video.txt file (it's the penultimate file). Find this line: "setting.mem_level" and edit its value to "0".
Example:
Save the document, right click on it, select "Properties", then tick the "Read-only" box. Select "Apply", then "OK" and that's it!
P.S. - If you ever want to change the resolution or the video settings, un-tick the "Read-only" box, launch the game, change the settings to whatever you wish, minimise the game, then tick back the "Read-only" box.
Example:
Save the document, right click on it, select "Properties", then tick the "Read-only" box. Select "Apply", then "OK" and that's it!
P.S. - If you ever want to change the resolution or the video settings, un-tick the "Read-only" box, launch the game, change the settings to whatever you wish, minimise the game, then tick back the "Read-only" box.