Lift battery out of housing starting at the end that was secured by the screws.

Watch out for the odd-shaped silver washers beneath the battery connector (see second photo); they are necessary for proper mating of the electrical contacts when the screws are re-tightened.

When reinstalling the battery, make sure the washers are in place. Holding the battery at an angle (see first photo), insert the two retaining tabs on the bottom of the battery into the slots in the case. Then rotate the battery into place and screw down the connector.