Add a new display resolution

Check available modes


Set a mode

xrandr --output DisplayPort-0 --mode 1920x1080  --rate 30

If you need to add a mode you know, your hardware is capable of but isn't listed, you can add it manually. In a terminal generate a modeline

cvt 3840 2160 30

The output should look something like this

Modeline "3840x2160_30.00"  338.75  3840 4080 4488 5136  2160 2163 2168 2200 -hsync +vsync

Add the new mode

sudo xrandr --addmode DisplayPort-0 3840x2160_30.00

And set it the same way as above

xrandr --output DisplayPort-0 --mode 3840x2160_30.00

Note that setting the resolution is not permanent. You will probably want to add the line to ~/.config/openbox/autostart.