You need to remove the trim strip above the vent.

There's a small screw on the lip of the trim in the wheel arch, a 10mm bolt inside the wheel arch, and a 10mm bolt inside the car, behind the trim that hides the seatbelt mechanism and rear speaker.

However be warned! One of the bolts in the trim strip is very long, so you need a deep 10mm socket to be able to get it off.

The ones Halfords sell as

'deep sockets' are deep enough.

Once that's done there's another 10mm bolt inside the wheel arch at the bottom, holding the side vent on.

It's in the bottom corner nearest the wheel.

HTH
