This is a big job, have a look here which will show you how to go about it.click here
1.Prepare the computer for disassembly (refer to Section 6.3, “Preparing the Computer for Disassembly,” for more information).
2.Remove the cmos battery (refer to
Section 6.24, “System Board,” for more information on removing and replacing the RTC battery.
If you go to page 6-51 this will show you the battery.