how do i remove the centre top air vent on my 56plate Zafira?

how do i get in to change the centre top air vent on my 56 plate Zafira? The one in the car is broken so I have bought a new one from Ebay but don't know how I get into it to exchange it for the new one. The vent I'm on about sits at the top of the dash and aim towards the windscreen.