For the two centre vents "I actually found these easier to fit than the two outer" I bent a metal coat hangar, and pulled the outer ring off first. Theres a little notch at about 12 0`clock. Hook the bent coat hangar on here and pull quite hard. This releases the top clip, then just pull all around the ring untill it comes off. I then got two bent pieces of coat hangar and hooked them through the centre of the vent, either side of the knob and pulled. Dont be afraid to pull hard, you really have to. I also kinda "rocked" the vent whilst pulling to
help persuade it to come out. It will eventually come out so dont worry. Then once it pops off, bend the four clips inwards, so they are facing out of the hole in the dash and again pull hard untill the whole module comes clear of the dash. Then its just a case of connecting the new pieces up to the module, and pushing the module back in. Finally once its back in, complete with new piece, you just click the new ring back into the holes. Isnt really hard to do, just a pain in the ass, but like i say dont be afraid to pull to hard cos you do have too!! Hope this helps!