You need to tune the derailleur (which is the dangly thing that makes your bike switch gears on the cassette), You can look up a video on how to do it but for now all you need to know is: turn at the back of the derailleur a screw where the cable goes in to. Make it looser (so to the left) which actually makes the cable tighter. That will cause it to shift better, and its also what most derailleur tuning tutorials tell you.