Warning : Comeen Play must manage the rotation and not the screen/player system (so do not change the orientation of your screen except in Comeen Play)
Use the "Displays" menu to go to the management of your screens and click on the screen whose orientation you want to change:
Click on the pen:
At the bottom of the form you will find a section "Chrome OS specific", in "Orientation of the screen" choose your orientation:
Click on Save
The change is instantly applied to the screen.