The battery is a Lithium cell CR 2032; available at Walmart, and other convenient stores (camera section).
After you open the computer side cover, hold the case to ground yourself and the computer, and make a note of the battery polarity. Use a small screw driver to remove the old battery, and then replace with the new battery. Pay attention to the polarity while inserting the new battery.
This battery replacement will reset the BIOS to factory defaults. Wait for 10 minutes before putting in the new battery.