If possible, drain the battery before disassembly. When the battery is charged, there's an increased risk of a dangerous thermal event if the battery is overheated or damaged during repairs.
If the rear glass is cracked, completely cover it with packing tape to contain the glass shards and avoid injury.
Prepare an iOpener and heat the back of the phone along its bottom edge for about two minutes, or until it's slightly too hot to touch. This will help soften the adhesive securing the rear glass.
You may need to reheat and reapply the iOpener several times to get the phone warm enough. Follow the iOpener instructions to avoid overheating.
A hair dryer, heat gun, or hot plate may also be used, but be careful not to overheat the phone—the display and internal battery are both susceptible to heat damage.
Apply a suction cup to the bottom edge of the rear glass.
Pull up on the suction cup with firm, constant pressure to create a slight gap between the rear glass and the frame.
If the glass is cracked, the suction cup may not stick. Try lifting it with strong tape, or superglue the suction cup in place and allow it to cure so you can proceed.
This may require a significant amount of force, but you only need to open a very slight gap with the suction cup to insert your tool.
If you have trouble, apply more heat to further soften the adhesive, and try again. The adhesive cools quickly, so you may need to heat it repeatedly.
Insert an opening pick into the gap you created under the rear glass.
Slide the pick all along the bottom edge of the phone to slice through the adhesive securing the rear glass.
Slow down and slice very carefully as you get to the corners. The curved part of the glass along the left and right edges can crack very easily if the pick pushes up against the curved glass.
After being cut, the adhesive will sometimes stick back together as it cools. To prevent this you can leave the pick in this edge after cutting, and continue the next steps with a new pick. Repeat this with each edge, leaving a pick and continuing with a new one.
Slide the pick all along the top edge of the phone to slice through the adhesive securing the rear glass.
Slow down and slice very carefully as you get to the corners. The curved part of the glass along the left and right edges can crack very easily if the pick pushes up against the curved glass.
After closing your device back up during reassembly, stack something heavy, like a textbook or two, on top of the device for 30-60 minutes. This ensures a strong adhesive bond.
Use the pointed end of a spudger to pry up the battery connector and disconnect it.
During reassembly, this is a good point to power on your phone and test all functions before sealing it up. Be sure to power your phone back down completely before you continue working.