Why do you need to replace the cassette? Is it worn or are you having a hard time on the hills?
Anyway, you need to replace the existing cassette with a compatible cassette. Most Shimano and SRAM cassettes are interchangeable, but not so with Campagnolo. Since your existing cassette has 8 cogs on it, you will need to replace it with another 8 cog cassette. These are fairly easy to find if you look in mountain bike parts. The big question is if your rear derailleur has the capacity to shift up to a 28 tooth cog. The capacity of the RD should be printed on the back of the parallelogram body. If it says that it will go to 28 teeth, you are good to go except that you will probably have to add a couple of links to your chain to accommodate the larger cog.