First, you need to find out how the screen that you want to rotate is labeled.:
xrandr -q
Output should be something like:
Screen 0: minimum 320 x 200, current 1366 x 768, maximum 8192 x 8192
LVDS1 connected 1366x768+0+0 (normal left inverted right x axis y axis) 344mm x 194mm
1366x768 60.1*+
1360x768 59.8 60.0
1024x768 60.0
800x600 60.3 56.2
640x480 59.9
VGA2 disconnected (normal left inverted right x axis y axis)
HDMI2 disconnected (normal left inverted right x axis y axis)
DP1 disconnected (normal left inverted right x axis y axis)
This will give you connected monitors It says connected next to the name. Determine the one that you want to rotate.
Next, we want to turn it around. For that, we use one of the following
xrandr --output <label> --rotate right
xrandr --output <label> --rotate left
xrandr --output <label> --rotate inverted
xrandr --output <label> --rotate normal
Of course replace with your monitor label in the above commands and you'll be able to rotate the screen.