To detach the screen:
1. Turn on the Surface Book. You cannot detach the screen while the device is powered off.
2. Press and hold the Detach key until it turns green. You will hear a click to indicate the detachment is complete. If you are logged in, there will be a display (typically in green) along with an audio sound that indicates the detachment is complete.
3. Hold down the keyboard with one hand and lift the screen with your other hand.
To reattach the screen:
1. Place the screen back on the keyboard dock and ensure it is seated firmly and evenly.
2. There is no audible sound for reattachment unless you are currently logged in.
Here is the official article from Microsoft on detaching your screen.