You should have been provided with a TSA approved key with a screw driver head in a small zip lock bag that came with the bag. Just open up the zipper for the internal lining and then remove the strip of tape and then unscrew the 4 screws holding the battery in place using that key. It is very easy! If you have misplaced this key then you can just use a regular screw driver of the correct size and that will also work!
Here a picture:
