Heat the back of the phone along the bottom edge. Once the adhesive is sufficiently warm, insert a pick or pry tool inbetween the seam of the back glass and the body of the phone.

Use the pick/pry tool to separate the back glass from the adhesive, starting at the bottom edge. Slowly move your tool around all edges of the phone, ensuring you don't insert too deep and sever any cables. It will likely be necessary to re-heat the other edges of the back glass as you are prying around the phone.