>>961397Shortening the chain will probably not help. The issue looks like the derailleur, not the chain. Can you see a model name of the derailleur anywhere on the bike?
>>961415I assume you are referring to rear shifting, not front shifting. Possible causes:
>Derailleur is not adjusted properly (see Park Tool link posted by other anon)>Derailleur hanger is bent>Components are worn (sharked teeth, stretched chain, or less likely, damaged derailleur)>Cables/housing in need of replacement>>961430Regardless of the issue, you can do it yourself, the trick here is, as the other anon said, diagnosing it. If you know exactly what's wrong then it's pretty straightforward. You could try to diagnose and solve it yourself, it would be good experience. First check for a bent hanger, see pic related. The derailleur pulleys should be verticle, just like the cassette cogs. If they are at an angle, your hanger needs to be replaced.
If your hanger is good, then use the Park Tool link to check your derailleur adjustment.
You can also check your cables and housing (this won't be a perfect test) by disconnecting the cable from the derailleur, holding it at the end, and then pulling the shift levers. See if it feels like it's sliding smoothly within the housing. Make sure not to fray the end of the cable when messing with it.
If you have issues with your front shifting, some of the potential issues are the same. But in this case you will not have a hanger to check, and you WILL have to check the derailleur's height and rotational alignment.
