Now that you can connect to your Nexus device from your computer it's time to unlock your bootloader. To do this you must first boot into your bootloader. You can either manually turn off your phone or tablet and hold down the power button and the volume down button to enter your device's Bootloader Menu or you can enter the following commands into your terminal or command prompt.
Run the following command to make sure your device is properly connected to your computer. If it returns a string of characters it means that you are all set to start updating your device.
./adb devices
Now to enter into the Bootloader menu just run the following command.
./adb reboot bootloader
At the bottom of the screen there will be several things listed including the lock state of the device. This should say locked unless you have unlocked your bootloader in the past and never went back and locked it again.