The Battery is held in its own compartment. As shown in the first photograph, both sides have tape over them that has to be peeled off.

Once the tape is removed, unclip the 4 clips and remove the plastic top cover. This will expose the battery as shown in the third photograph.

The Battery can then be unplugged and removed.

If you already have your replacement battery, you can fit it and reassemble in reverse order. Otherwise, store all of your parts safely whilst you source a replacement battery.