Carefully pry the cover away from the phone.

First use tweezers to peel away any remaining adhesive from both the back cover and the phone's chassis.

Then, clean the adhesion areas with high concentration isopropyl alcohol (at least 90%) and a lint-free cloth. Swipe in one direction only, not back and forth. This will help prep the surface for the new adhesive.

If it didn't already fall off, lift the metal camera plate away from the back cover. Carefully apply adhesive to the cover. Replacement adhesive comes in a pre-cut sheet to match the exact contours of the back cover. If that's not available, you can use a high-bond double-sided tape, such as Tesa 61395. With the new adhesive applied, reinstall the camera plate.

After installing the rear cover, apply strong, steady pressure to your phone for several minutes to help the adhesive form a good bond.