Before buying a new trip display you will need to find out what type you have so its essential you take the trip out first. On the back of the trip is a connector it will either be grey or yellow, you must pick the right colour when replacing it. Also be careful when buying trips from vectras and the like because some of them are too big. The one in my car was too big so I had to modify it to fit.
Take out the screw in between the steering wheel and the speedo clocks - you may need to take the plastic cover around the steering wheel where the ignition is to get at it.
Prize out the headlight switch by using a small flat screwdriver (you might need two screwdrivers, one for each side). I usually put tape on the ends of the screwdriver to try and avoid scratching the plastic. There are two bits of metal at each side which you push in to get the switch free.
With the door open take off the trim panel to the far right of the headlight switch it should just slide out. You should then be able to see a screw behind where the headlight switch would normally be. You might need a torch to see it, take it out.
Prize out the vents above the centre console, take care not to scratch them although they should pop out easy. You will then see the screws you need to take out, think there are three screws there.
Now CAREFULLY slide out the speedo clocks and disconnect the trip display.