1. Install xrandr and lxrandr
2. Open the script with your favorite text editor. Change “1680×1050”, and or 1680×1050, and or 1680×1050 60 (60 been the frame rate, give extra attention to that! It’s very important that you type the correct frame rate for your external monitor!) by the resolution you want to output on your external monitor. Save and exit.
3. Give execute permission to the script. Run it. Enjoy :)