The activated carbon in the filter will need to be changed every 2-3 months, you can purchase a box of Activated Carbon here, it will last you 1-2 years.
The tray containing the activated carbon can be found inside the filter, unplug the filter, remove it from the water, dry it off and then pull apart in the centre.

The activated carbon is stored inside the white tray. This can be pulled out of the filter and opened up to replace the carbon. When replacing your activated carbon make sure you rinse the media basket through until the water runs clear.