iFixit

Edit Step 16  ¶ 

• Use a spudger to pry the battery up from the rear panel. The battery is attached with an adhesive strip around the perimeter of the battery. To prevent the battery from bending during the removal process, we recommend against using just the plastic pull-tab .
• The iPhone's battery is attached to the rear case with a mild adhesive. This adhesive usually comes out attached to the battery. If your replacement battery already includes an adhesive backing, you're all set. If your replacement battery doesn't have an adhesive backing, you'll want to transfer the adhesive from your old battery to the new one.
• You can use a hair dryer or heat gun on its lowest setting to heat the back of the case. This will help loosen the adhesive bond before using the spudger to separate the battery from the case back.