Place the Roomba on its back and fully loosen the four (4) bottom screws. Don't remove them completely; they're permanently attached to the cover.
Then, remove the one (1) spinning brush screw and pop the brush off.
Finally, remove the bottom cover.
As always, reassembly is the reverse of disassembly. While you're at it, go ahead and replace the spinning brush. Some replacement battery kits, like this one, come with an extra brush. If yours didn't, you can grab one on Amazon for a few bucks.