get a little screwdriver, put it in the little hole near the key ring loop and twist it, the fob bit "should" pop off then break the other in half and bish bash bosh ... :)
 
***when i say break i mean just pull it apart, or you might be able to get battery out without pulling the top off
 
i aint got a little hole next to where the key ring loop is, which side comes off the badge side or other side?

Open the key blade and inside is a small hole. Use a flat bladed screw driver and twist it in the hope to prise the case open a little. The battery is in the bottom loop half of the fob. You then simply pop it open to get to it. :D