The battery is secured with three pieces of adhesive, a removable pull-tab in the center, and on the SIM tray side. A third piece of adhesive on the button side has no pull-tab.

Pull the adhesive tabs in a slow constant movement to free the battery.

Should those adhesive tabs snap, you might want to use an iOpener to heat and soften them a little bit so you can pry out the battery with a spudger.

There are two cables running directly underneath the battery. Take care not to damage them when prying with a spudger! You can see their position in the last picture.

Slowly lift the battery toward the button side and free it from the remaining hidden piece of adhesive.

When reassembling your phone, replace old adhesive with stretch release adhesive strips, double-sided tape, or pre-cut adhesive strips.