Remove the back cover.

To re-install the back cover:

Use tweezers to peel away any remaining adhesive from the phone's chassis. Then clean the adhesion areas with high concentration isopropyl alcohol (at least 90%) and a lint-free cloth to prep the surface for the new adhesive. You don't have to clear out adhesive down to the plastic but larger pieces should be removed.

Turn on your phone and test your repair before installing new adhesive and resealing the phone.

Carefully apply the new adhesive to the back cover, then line up one edge of the glass against the phone chassis and firmly press the glass into the phone.