Carefully lift the lower case from the upper case enough to allow access to the internals of the Apple TV.
Be aware that the hard drive is physically attached to the lower case and that a ribbon cable runs from the drive to the logic board in the upper case.
Carefully pull the hard drive data cable straight up from its socket on the logic board.
During reassembly, make sure the hard drive data cable is firmly seated in its socket on the logic board; otherwise, during boot-up you will see a flashing question mark "?".