Power off, and pull out your MPU board. I have the same machine as you, and never had to replace the battery so I'm not sure of the voltage... likely 3 or 3.6v lithium cell. The battery should be on a smaller card, probably soldered in. Look at our vendor page - usually Jim, KLAR or Slot Rush may carry it. Or just check eBay or Amazon.
So you'll need to carefully unsolder it, and solder in the new one. When you put the MPU back in, make sure it's seated ALL the way in the socket.
I'm not home now but I found this on the forum. Your board should look similar or the same.