1.Find the barrel adjuster on your rear derailleur. This is a small knob located where the cable housing comes into contact with the rear derailleur.
2.Give the barrel adjuster a half turn in a counterclockwise direction. This will take up a small bit of slack in the cable. ( Image a.)
3.Try shifting. Problem solved? If not, give it another half turn. ( Image b.)
4.Give up to two full turns. If the problem is still not solved or your shifting is now much worse, turn the barrel adjuster back to its original position.
5.Try turning the barrel adjuster in the opposite direction. This will put more slack into the cable. ( Image c.)
6.Work in half-turn increments, as before.
7.Return the barrel adjuster to its original position if you are unable to improve your shifting this way. It's time to go to the bike shop.
It's even easier to adjust the cable tension if you have a bike stand. Turn the pedals forward as you turn the barrel adjuster slowly. Listen for the clicking to diminish.
Bad shifting can have many other causes: a worn chain or cassette, a misaligned derailleur hanger, worn cable or cable housing, a damaged or poorly adjusted derailleur. If you still have serious shifting problems, take your bike to a bike shop.