Is there a release somewhere to open the trunk so that I can access the battery?

There are a couple things to try, First take the manual key, insert it and turn it counterclockwise and push it in. If you have the correct key, it should unlock.
However, another method would be to apply power to the vehicle from under the hood.
Take the new battery, set it on the ground or other surface, open the hood and connect the battery to the car with a set of jumper cables. Use the positive side to connect from the pos battery terminal to the battery stud on the back of the alternator or to the jump terminal located on the driver's side terminal block. Then connect the negative cable between the battery negative and a clean ground point under the hood like the engine block or a jump stud if it has one. That should supply enough power that the doors can be opened, and the trunk unlocked.

You're welcome. I've used that trick a few times on vehicles like that.