These are the instructions to install TWRP recovery on Nexus 9 running Android 6.0 Marshmallow.

REQUIREMENTS

• The following tutorial is made only for the Google Nexus 9 tablet. Do not follow this guide unless you have the Google Nexus 9 or else you will brick your device.
• The following method hasn’t been extensively tested and it is created with the assumption the previous exploit is not yet patched by Google. With that knowledge, you shouldn’t follow this guide unless you are an experienced Android user and are confident getting out of boot loops.

BEFORE WE BEGIN

• Just like getting root access on the Google Nexus 9, installing a custom recovery on your device will and does void the warranty. You can usually get the warranty working again by installing the stock recovery and wiping the custom recovery at the same time. By flashing a stock recovery you are automatically removing the custom recovery, so you do not need to worry about trying to wipe the custom recovery. All you need is to work out how to flash the stock recovery — and we don’t show you that in this guide.
• Our guide for settings up ADB and Fastboot works for Windows PCs only. Therefore, you can only follow this guide if you have a Windows computer.
• Double-check that the USB Debugging Mode is enabled on your device before you attempt connecting it to the computer. You can test that out now by heading over to the Menu > Settings > Developer Options > USB Debugging Mode.
• Stay within the Developer Options menu and enable the OEM unlock option.
• All of you should have the Developer Options menu from the Settings. However, if you do not have that, you can unlock the hidden menu by heading to the menu > Settings > About Device > tap the build number until it says you are now using the device as a developer. You do not need to worry about hiding that menu again — you are still using the device in normal mode, but it’s just calling you a developer so you sound cool. That being said, there are ways for you to hide the Developer Options again if you must — Google is your friend for that.

HOW TO INSTALL TWRP RECOVERY ON GOOGLE NEXUS 9 RUNNING ANDROID 6.0 MARSHMALLOW

1. Download the TWRP recovery for the Google Nexus 6 from here.
2. Follow our guide for setting up ADB and fastboot on your Windows computer.
   Completely power down the Google Nexus 9 tablet by holding down the Power key until it’s off.
3. Reboot the Google Nexus 9 tablet by holding the Power + Volume Down.
4. Once your tablet is in the bootloader mode, connect it to the computer with the USB cable.
5. From within the same folder as your ADB and Fastboot, right-click the mouse and hold down the Shift key on the keyboard to bring up a new menu on the computer.
6. Select the ‘open new command window here’ from that new menu.
7. Type the command: fastboot flash recovery twrp-2.8.7.0-flounder.img
8. Now choose the recovery Mode option from the Fastboot menu and your device will boot into the custom recovery you just installed.
9. Now you can learn how to root the Google Nexus 9 tablet running Android 6.0 Marshmallow (MRA58N).