Use a #00 Phillips head to unscrew the five screws fastening the battery to the board. Make sure to have a little jar or something to collect the screws -they're fairly tiny.

Also make sure that the black/white twisted wire is free from the battery housing - it snakes around the bottom edge of the battery and may be caught up in the plastic.

Remove the battery and dispose of properly (i.e. at a municipal battery/electronics recycle center).