Welcome to the forum, gunnysack! Removal of the battery is pretty simple.
Look for a metal, U-shaped rod that goes over the width of the battery and then down both sides. UNDER the box, you should see those rods go through the bottom of the box and through a metal plate which is held to those rods by two nuts. Simply remove those nuts and you will be able to lift the metal hold-down rod off the top of the battery.
Then, you can disconnect the battery cables (negative cable first) and that frees the battery. Lift it out of the box and replace it. Reattach everything in reverse order. Attach the positive battery cable before you attach the negative cable. Removing/attaching the cables in the order I'm specifying helps prevent an accidental short circuit with a wrench that would generate a huge spark.
- Jack