Switch off your phone.

Heat an iOpener to loosen the adhesive beneath the rear glass. Apply the iOpener for at least two minutes. If you have a heatbed with precise temperature control (like any FDM 3D printer with heatbed) 45°C is enough. You might want to repeat this step several times during the rear glass removal.

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.