Giant Cypress Upgrade
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.