iFixit

Starting with the battery cell closest to the dock connector, run a plastic opening tool underneath the edge of the battery closest to the logic board void to make enough room to insert the flat end of a spudger

Run the flat end of a spudger along the two long sides of each battery cell to completely separate them from the adhesive securing them to the rear case.

If the adhesive is very difficult to separate, reheat the rear panel and try again.