Lift the back cover up and remove it.

If the back cover is still stuck to the frame, slide an opening pick around the entire perimeter until the cover separates completely.

During reassembly:

This is a good point to test your phone's functionality before sealing it up. Use this guide to run a hardware test. Be sure to power your phone back down completely before you continue working.

Apply new adhesive to the perimeter of the underside of the back cover after cleaning the relevant areas with isopropyl alcohol (>90%) and a micro-fiber cloth.