Insert the point of a spudger underneath the black battery pull tab on the left edge of the phone.

Slide the spudger upwards along the edge to loosen the pull tab from the battery.

The battery is held in place with three separate stretch adhesive strips, which are connected to a single black pull tab. You can try to pull all three strips out together, but it is easier to cut the black pull tab where it is notched and pull each strip out individually.