During firmware update always connect the USB while doing the updating process. To begin you will need to reset the RT.
To turn off the RT please press and hold the button down for 5 seconds. If the unit does not shut down then please try this method, open up the RT case to gain access to the board and short both C4 and R1 together like in the photo. This will completely turn off the RT, give it a minute and press the power button until you get the pulsing magenta LED and when completed you can go back to Android 7.