After putting a few miles on your bike the cables will stretch as they wear. The result is less precise shifting. You’ll often hear a clicking noise from your chain wanting to jump up or down a gear on the rear cassette. This can typically be fixed by adjusting the tension of the cable that runs from your shifter to your rear derailleur. Depending on the style of your bike, there will be barrel adjusters located on the shifters, cables and/or near the rear derailleur. Use the barrel adjusters to fine-tune the cable tension.
Turn the barrel adjuster ¼ turn at a time. Turn the adjusters clockwise if the chain seems to want to fall into a smaller cog. Turn the barrel adjuster counter-clockwise if the chain won’t shift up to the larger cog.
If you’ve turned the barrel adjuster more than a full turn and it hasn’t solved the problem, you may need to adjust the cable tension near the rear derailleur itself by loosening the cable anchor so you can pull the cable tight. Be sure to turn your barrel adjuster all the way tight when you do this so you have room to adjust it later when your cable stretches again.