The Discovery Sport cabin air filter is located behind the dash, on LHD versions, it is accessed from the passenger foot well.
To replace the cabin air filter you will need to remove three panel clips that hold a panel in place underneath the glove box.
To remove these clips you will need to use a plastic pry bar to gently compress the clip. You will first apply pressure to the large side of the clip and this will pop it out a little bit of its hole. You will then need to hold the clip out as your pry on the small side of the clip to completely remove it.
The cabin filter is bendy and flexible so you can squish it in place.