Grab and remove the back cover.

If the back cover is not lifting easily, use an opening pick to separate any adhesive that may have resealed.

During reassembly:

This is a good point to power on your phone and test all functions before sealing it up. Be sure to power your phone back down completely before you continue working.

Retaining water resistance after the repair will depend on how well you reapply the back cover adhesive, but your device will lose its IP (Ingress Protection) rating.

Use tweezers or your fingers to remove all adhesive from the back cover and the phone. Use heat or isopropyl alcohol (90% or greater) and a coffee filter or lint-free cloth if you're having trouble removing the adhesive.

If you're using custom-cut adhesives, follow this guide.