You can either search eBay and pay around £7.49 for a replacement CMOS battery or you can remove the depleted battery from the connector and connect up a replacement like I have done here...
The first thing you need to do is remove the keyboard by pressing the 2 keyboard release catches as shown here:
The CMOS battery is hidden behind the panel as shown below:
Disconnect the CMOS battery from the laptop.
Carefully prise the positive and negative wires off of the battery so it looks like this:
Now you need a way to secure the wires to a new replacement CMOS battery, I tried solder but that didnt work so I grabbed some Sugru instead...
Take a small amount of the Sugru and roll it around in your hand until it becomes supple and then use it to secure the metal contacts to the appropriate side of the replacement CR2032 battery
It takes 24hrs for the Sugru to set and then you should be good to go!