I had a need to reinstall the image in my Nexus 9 tablet. the recommended way was to go through the fastboot. Download the images from the google website. Then get the tablet to the fastboot and then install the software.
Make sure that you have adb and other tools installed in your computer. With the debugging enabled the
$adb devices
should give you as below
List of devices attached HT4AJJT03366 device
should list your devices, if it is says unauthorized then you need to accept the dialog box “Allow debugging” in the tablet, that you authorize the computer to access the tablet.
Now issue the command
adb reboot recovery
This will take you the HBOOT or fastboot screen.