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.
There are two tabs on the old battery. Lift on them to pull the battery out of its housing.
The new battery should be identical to the old one. Drop it in place.
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.
Charge your Roomba overnight to give him a good night's sleep.
Your robot friend will now be back to his old self. Pat yourself on the back and feed him some crumbs: