You can downgrade your Nexus 5X, Nexus 6P, Nexus 6 and Nexus 9 to Android 6.0.1 Marshmallow from Android N, either by manually installing the latest public version of stock firmware package provided by Google or by requesting for the latest public version of OTA (over the air) update. Both methods will wipe all data on your device, so you need to backup all data before proceeding.
If you have enrolled for the Android N Bata program, you can opt out for the program to get the OTA update for the latest stable public version of Android (Android 6.0.1 Marshmallow). Just head over to the Android N Beta program page [HERE], click on the green button ‘UNENROLL’ (next to your device) and click ‘Leave Beta’ button on the pop up window.
Google will send an OTA update that will install the latest stable public version of Android (Marshmallow) on the device. It will wipe all data on the device, so backup all data before leaving the Android N Beta program.
If you have not enrolled for the Android N Beta program and installed the Android N by downloading the factory image, you can repeat the same procedure for downgrading your device. All you just need to head over to the Google Developers page [HERE], download the required factory image and follow the instructions there to manually revert to the latest Marshmallow build.
For detailed manual installation instruction, follow this guide.