Apply a suction cup to the back cover, as close to the center of the bottom edge as possible.

While securing the phone with one hand, pull up on the suction cup with strong, steady force to create a gap between the back cover and the frame.

Insert an opening pick into the gap.

If your back cover is badly cracked, covering it with a layer of clear packing tape may allow the suction cup to adhere. Alternatively, very strong tape may be used instead of the suction cup. If all else fails, you can superglue the suction cup to the broken back cover.