Replacing a bike chain is very easy.
Maintenance is making sure the chain is cleaned and lubricated as often as possible. It will always wear during use, but the life of the chain can be increased dramatically with a little lube.
If the current chain hasn't already broken, it will need to be with a chain cutting tool. The new chain should be cut to the same number of links as the previous one. Now's a great time to take note of how much that old chain has stretched!

The chain stretches over time by the links cutting into the rollers simply during use. Changing gears on some bikes isn't such a smooth transition, which speeds up wear. The links biting into the rollers cumulatively results in the chain stretching out a couple of millimeters. If your chain checker doesn't fit, it's time to replace the chain.

Wear isn't limited to the chain. The sprocket teeth (front and rear) wear down as well, and not having sharp teeth to grab the chain can lead to chain slippage or even missing a shift. If you happen notice a flaw in the chain, it could be from a bent link. The failing link can be found just by slowly pedaling backwards and listening to the chain pass through the rear derailleur.

With the new chain measured and cut to length, it's time to feed it through the drive train.

Route the chain through the rear derailleur and onto the largest rear cog, but let it hang around the bottom bracket to give some slack to make the connection.
Chains are joined in 3 ways:
- Special pin - Newer chains will have a reusable pin for removing and installing the chain for cleaning
- Master link - unique link that is snapped on
- Any pin - Chain ends are connected by putting a regular barrel roller through two links to complete.
When the chain is joined, make sure the joining link is bending properly as all the others. You can now feed it onto the chainring in the front, and the bike is ready to go!