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.
Wow, looks like a lot of people harping on one point & not offering much other helpful advice. While probably right that you are not going at as high a cadence as you think, they are ignoring that the geometry of your body on the bike absolutely has an effect on your ability to maintain a a higher speed & cadence.
More below, but to go faster I suggest: Raise your seat, lower your stem & stem angle, put a set of butterfly bars on in an inverted position.
I have a Giant Cypress Hybrid also. The bike naturally has a very upright position, creating two issues working against you. 1: Much more wind resistance than a a bike with drop or flat bars. 2: Your weight & angle over the pedals is shifted back much more which is also less efficient & makes powerful, prolonged pedaling (higher cadence) more difficult.
The first thing I did when I wanted to start going faster on this bike was lower the angle & height of the stem & flip the handel bars upside down (left to right, not rotating still mounted to the stem), I also raised the seat as much as i could while still leaving enough bend in my knees while extended. Doing this made noticeable increases in my ability to pedal harder. Later on I experimented with tri-bars (could gain 2 mph on a straight away after dropping onto these), now I have switched to inverted butterfly bars, which while maybe not providing as efficient a position as the tris, they are, for me, more practical.
With the gearing on this bike, personally I've never found myself needing a higher gear (unless flying downhill at 38 mph, and then well, I really don't want to go any faster anyway...) I have found myself wanting some lower gearing though, but that's another story.
Hope this is helpful.
Go to you local bike shop. See if they have something (used will do just fine) with a cycling computer on it that has cadence measurement (any cycling computer). Ask to take it for a test ride, and get to where you feel your legs are spinning at about the same rate they normally are on your bike. Then look at the cadence read-out. About 90 RPM is what you're shooting for. If it's telling you that you're not pedaling that fast, downshift (smaller chainring, bigger rear cog), until you're hitting that number. Then, when you can get your legs moving that fast in your 48/11 on your bike, you can start worrying about taller gears.