I need to locate cmos battery on my laptop?

i have a hp pavilion ze4540us and ive been told that i need to replace my cmos battery, can someone please tell me how to find it so i can change it, or do i have to take it to someone else. thanks|||its an oversized watch battery. the biggest flat round shinny piece of metal on the entire mother board, dont mess with anythign thats got fins on it|||Just open up the computer (all the way) and look for one of those flat watch batteries.