You need to remove the equatorial head and access the interior or the pier.
Remove the three large hex head bolts and lift the heavy head off the pier. Inside the pier is a aluminum plate. The plate floats in the pier with L-shaped brackets maintaining its position within the pier’s cylindrical main housing. There is a circuit board just under the plate and screwed to it. Don't disturb this board. Instead, carefully lift the plate with either a small screwdriver, hook or stick a piece of masking tape on the back of the plate in order to gently pull it up.
The battery is located under a clip on the PC > Autoguide > Power Cord board inside the pier. Replace with a 3-volt CR2025, available at many drug, photo and hardware stores.