Chains are where you can get a lot of grit and just plain dirtiness accumulating so it’s something you should clean and maintain on a regular basis. If you need to install a chain, remember to get one that is compatible with your system because the larger the number of sprockets you have on your bike, the thinner the chain you will need. A thin chain for a 10-sprocket bike can theoretically be used on sprockets designed for wider chains.
The inverse is not true. You cannot use a chain from a single-speed bike (wider sprocket) on a 10-speed (thinner and multiple sprockets).
First, to install a Shimano bike chain, you’ll need to size the chain. To do that, you’ll need to click the shift levers so that the front derailleur is on the largest chainring and the rear is on the smallest sprocket. Thread the new chain over the big sprocket and chainring. You don’t need to thread it through the derailleurs just yet. Measure off where the chain meets and then count two pin-holes of extra chain links. REMEMBER: you must cut the chain so that it ends with the inner plates and not with the outer plates. If the measurement lands right on a link with outer plates, cut the chain one link up so you end up with inner plates and not outer ones. Shimano uses a special connecting pin that has a break-away guide that comes with the chain. This is what you’ll use to connect the chain, once it is threaded on the chainring and the derailleur.
Use your chain break tool to cut the chain and then proceed to properly thread your chain through your derailleurs. Now, with your chain tool, press the connecting pin into the link. The break-away guide will either break off or you will have to snap it off with the chain tool.