Hard Reset HTC Nexus 9 LTE

First method:

1. Turn off your device.
2. Press and hold  Volume Down + Power at the same time for a few seconds.
3. Release held keys when you see Bootloader Mode.
4. Then press Volume Down to select option "Factory reset", and press Power button to confirm.
5. Well done!

Second method:

1. Turn on the phone by using the Power button. 
2. After pressing the Home button go to Settings and then to Security.
3. Choose the Factory Data Reset.
4. Select Reset Phone. 
5. Tap Erase Everything if you are sure you want to clear all your data. 
6. Well done!

WARNING !
Hard Reset will erase all of your data
All described operations you are doing at your own risk.