You have to remove the top casing, usually starting with the top bezel, then the keyboard. The CMOS battery is located on the motherboard, so, dending on the design of the case you might have to get through a couple of layers before reaching it. Also, there will be a few small ribbon cables to be careful of (mousepad and keyboard), the screen cable and possibly some hidden screws. Sometimes when you manage to get acess to the battery, you find that it's been soldered in. For those reasons, I would advise, if you're not too comfortable with it, that you take it to your local computer repair shop and let them do it for you. They'll probably charge around £20 to do it.