Find a sliding button on the lower back side of the device and with your finger, slide it to the left firmly.
Unscrew the four Phillips 00 screws on the four corners under the battery to unfasten the back case from the phone.
Pry the case off using a plastic opening tool.
Continue working the plastic opening tool around the sides of the device, unhooking the eight tabs securing the back cover to the rest of the phone as shown in the second photo.
The back speaker will be connected to the motherboard, and must be disconnected to remove the case.