With skill and dexterity, the battery should snap out. Just study it, and you'll figure out how to get it out. Don't force it, though. It may be soldered in. If its a new board, you may only need to move a prong to take the battery out.
Many newer boards have a small, flat coin-shaped battery which is a lot easier to remove.
Get a replacement battery. Take the old one to the store and match them up. It should be pretty easy to come by. Once you find the correct battery model, you may want to write it down somewhere. Put the new battery in.