Step 1 - On the bottom of the laptop, remove the two torx headed screws holding the bezel in place that gives you access to remove the keyboard
Step 2 - Turn the laptop back over and use plastic spudger (or carefully use a flat blade (slot) screwdriver) to remove the plastic bezel holding the keyboard in place
Step 3 - Carefully remove the keyboard and then unscrew all the screws as indicated in the photo below
Step 4 - Carefully remove the metal cover to reveal the motherboard, CPU and optical drive
If you look closely you can see the CMOS battery hiding above the optical drive:
Step 5 - Disconnect the CPU fan and place the metal housing to one side and then carefully flip over the laptop and remove the 2 screws holding the optical drive in place
Step 6 - Turn the laptop back over to gain easy access to the CMOS battery! If (like my laptop) there is a cable in the way of the battery, unplug it!
Step 7 - The button cell/battery that you need is a CR1220 - luckily I have a wide selection of button cell batteries so I was lucky to already have one ready to use...
Insert the new replacement CMOS battery in to its holder
Step 8 - Plug back in any leads you unplugged, insert the optical drive (and screw it in to place), reconnect the CPU fan and screw the metal cover back in to place, connect the keyboard, clip the bezel back in to place, flip the laptop over and screw in the remaining screws
Step 9 - TIME TO TEST (pun intended) - turn on the laptop and press F10 to enter the BIOS
Step 10 - Set the time and date and then Exit and Save from the BIOS
Step 11 - Power on the laptop and boot in to Windows!
BINGO! It now boots without any CMOS battery errors :)