Place your thumb on the upper case, inside the empty battery slot, and your index finger near your thumb on the left side of the lower case.
Pinch your thumb and index finger towards each other to release the clip. Without releasing pressure on the pinch grip, use your index finger to push the lower case upwards.
Orient the computer so the plastic clip in top left corner of the lower case is facing you. Release the clip by lifting the retainer. You may need to adjust the position of the hard drive to remove the retainer.