I'd like to downgrade my Nexus 9 to android 4.X (KitKat). Is this possible? I'm not the pro doing something like that and I could not find something with google.
Can you help me please? Thank you
It is unlikely, since the Nexus 9 was a Android 5.0 Lollipop launch device. There are no official (Google developed & distributed) versions of Android 4.4 KitKat for the Nexus 9.
Third party roms would be the way to go - but I have found no KitKat roms for the Nexus 9, and I personally doubt one would be developed unless there was a massive interest in it.
This is how you can do it: http://thetechrism.com/downgrade-lollipop-kitkat-nexus/
You can't do it without doing some sort factory image/reset. Google has not officially distributed version of KitKat, however. So you'd have to use third party.
You can find the easiest tutorial to downgrade your device here : Downgrade Lollipop to KitKat
You have to wipe/ factory reset the device and flash the Factory Images to downgrade the Android version.