Giant Cypress Upgrade I have very little knowledge when it comes to bicycles and how they work. With that being said, I have a Giant Cypress Hybrid. It is a great bike for the most part. I'm looking to get a little more top end speed out of it. When I'm on a paved trail, its simply nkt fast enough for me. Another gear or two would be great. Is it possible to just replace the rear cassette,  or do you have to do something to the front as well?
What others are saying is right.  If you have a 48/11 and are only going 20 mph (32 km/h) then you aren't spinning fast enough. You also won't be able to find a cassette with smaller than an 11 tooth cog.  You'll have to get bigger chain rings.  That being said, it might be difficult to spin fast on a hybrid with platform pedals. Personally I find it easier to spin fast when I have clipless pedals. Also, make sure the bike is fit correctly to your body. If your seat isn't high enough and your legs are bent too much, it can be hard to get a high cadence.