Step 1 - Unplug from the mains and remove the battery
Step 2 - Remove the hinge cover (make sure the screen is flat when you do this)
Step 3 - Use a plastic spudger (or carefully use a flat blade screwdriver) in the notch on the right-hand-side of the hinge cover to gently remove it
Step 4 - With the hinge cover removed you can now unscrew the keyboard:
Step 5 - Carefully remove the keyboard and the CMOS battery should be revealed.
Step 6 - Once you have replaced the CMOS battery (it's basically a CR2032 by the way) all you have to do is reverse the above steps and you'll be good to go!
Make sure you press F2 to enter the BIOS and check/set the date and time after replacing the CMOS battery as it's this battery that keeps the RTC (Real Time Clock) going when the laptop is powered off.