iRobot Roomba 5xx Series Battery Replacement

iRobot Roomba 5xx Series Battery Replacement

I want to replace the battery pack on an iRobot Roomba 500 series. How do I gain access to the battery pack already installed in the Roomba? I have seen photos of a battery pack purchased from batteryspace.com intended for the Roomba, but not photos showing how to access the battery compartment itself. Thanks, Bob Cochran