You have to break the key fob in half (with the help of a flat bladed screwdriver) one way then split the bit that holds the battery. Both need a bit of force but they do come apart.
I would also make sure that you have the battery available before you take the old one out. It's something like a CR2330 but can't recall exactly. Will find out. Can get from Maplins for £1.99 rather tha daft main dealer price.