To replace a mountain bike chain, you’ll need a chain-tool. Some chain-tools have removable bits that are different sizes for different link-pins. Most mountain bike chains have the same link-pin size, so do not be concerned if your tool only has one bit. The Park Chain-tool is a good choice, as it has a large handle and two chain-placement channels.
First, clean any excess dirt or grease off your chain. Do this by holding a rag lightly moistened with chain-degreaser around the chain, beneath the bottom-bracket. Work the chain through your degreaser rag by backpedaling, and make sure you grip the rag lightly.
After you’ve degreased & cleaned the chain, it is now easy to see the pins on your chain. Work around the chain, looking for the “Connector Pin” that some chains have. This pin will appear different than the rest, and will not have “peening” around the head. On some chains (Shimano), the pins are peened, which flares the head of the pin, adding extra strength to the chain. With Shimano chains, a pin which is pressed out will sheer off the peening, which weakens the chain. These pins with sheered-off peening must be replaced with “connector pins”.
Connector pins are long, with a tapered end & a peened end. After inserting the connector pin fully into the chain link, the tapered end is broken off.
When selecting a pin to press out, do not select either a connector pin, or the pin next to it on the same link!
On other chains (SRAM), look for a “Master” or “Quick Link”. This is a section of chain that is slotted, allowing you to separate the chain by sliding the links together. Reassembly is the reverse.
Place the chain inside the channel on your chain tool, making sure it’s fully seated inside the cradle.
Then turn the press handle clockwise, until the removal bit contacts the pin. Make sure the bit is on the center of the pin; then continue, pressing the pin fully through the link and out the other side.
Now that the chain is separated, and replacing the chain is the reverse, except for a few steps. Make sure that the chain is routed correctly through the rear derailleur pulleys, and through the front derailleur as well.
If the chain had peened pins, the “connector” pins are used at this point to rejoin the chain. Slide the connector pin into the chain, tapered end first. After the peened end is even with the other pins on the adjacent links, break off the tapered side with a pair of pliers.
On a “master” linked chain, grasp the two ends of the chain and hook them back together with the sliding links.
Run the chain through several cycles, looking for stiff links. If one is located, observe the link pins for uneven protrusion. If it is the pin which was just replaced, use the chain tool to evenly seat the pin so that it is the same as it’s neighbors. Chain link failure is often the result of protruding pins or bent plates.