To replace the brush, you'll need to turn the Roomba upside down.
Use a flat head screw driver to unscrew the brush. You may need to hold the brush in place with one hand to keep it still.
Note: A phillips head screw driver can work as well.
With the brush unscrewed, pull the brush assembly away from the robot to remove it. The screw will remain attached to the robot.
Grab your replacement brush, and screw it into place. Notice the new brush comes with its own screw. Don't attempt to over-tighten. Once the screw is in properly, it will be very difficult to keep turning.
Quick tip! I've linked to the official replacement parts kit for this vacuum, but you can also try a much cheaper, off-brand kit as well.