Apply a few drops of isopropyl alcohol (90% or greater) under the battery and let it flow around the adhesive to help weaken it. High concentration isopropyl alcohol acts as a solvent and dries without leaving any residue, so it will not hurt your iPhone.

Carefully wedge a plastic card under the battery on the side nearest the logic board.

Slide the card from the top of the battery to the bottom, pushing toward the edge of the case.