Inspect the top of the door for an anti-lift device. This device will be a wedge-shaped piece of metal, wood, or plastic attached inside the upper track of the door frame. It could also be attached to the top of the sliding door. Remove the screw from the device with a Phillips screwdriver and then pull it out of the door frame or out from atop the door.
Lift up on the door and see if the bottom can come out. If not, locate the adjustment screw in the bottom backside corner of the door and turn the screw counterclockwise to lower the door out of the top track of the door frame.
Lift the door up off the bottom track and tilt the bottom out away from the track so that you can remove the top of the door from the top track.