Using an infrared thermometer heat the surface to 170-180° F (roughly 70-80° C). This will loosen the glue but not damage the electronics.
Hold the gun at about 3 inches from your phone (in this picture it looks like its right on top). With the heat gun set to low, heat up the borders for about 3 to 5 minutes depending on the strength of your heat gun.
You must be careful when using the heat gun on this device. If the device is heated up too much then it may melt the digitizer and cause discoloration. If you start to see any discoloration then shut off the gun immediately.
If you are working with a hair dryer, it might take up to 1 hour.