Apply a heated iOpener to the rear glass for two minutes to loosen the adhesive beneath.
A hair dryer, heat gun, or hot plate may also be used, but be careful not to overheat the phone—the AMOLED display and internal battery are both susceptible to heat damage.
Apply a suction handle to the right edge of the rear glass.
If the back cover is cracked, the suction handle may not stick. Try lifting it with strong tape, or superglue the suction handle in place and allow it to cure so you can proceed.
Pull on the suction handle to create a gap between the rear glass and the frame.
Apply new adhesive where necessary after cleaning the relevant areas with isopropyl alcohol (>90%).
Secure the new back cover with pre-cut adhesive or double-sided adhesive tape. After installing the back cover, apply strong, steady pressure to your phone for several minutes to help the adhesive form a good bond—a stack of heavy books works well.