If you need to shorten or lengthen a chain on a bicycle or motorcycle, use a chain breaker tool. These little gadgets allow you to pull chain links apart without damaging the chain. Some chains have a quick release link that doesn't require the use of a chain breaker tool, but if all of the chain links look exactly the same, you'll need to use the tool. Chain breaker tools can be found at your local bicycle supply shop.
Unwind the handle of the chain breaker tool until you have enough room to place one of the chain links on the outermost slot of the tool. It doesn't matter which link, because they are all the same. Back the handle out until the pin is out of the way.
Turn the handle of the tool clockwise until the pin of the tool connects with the pin in the chain link. Continue twisting the handle to push the pin in the chain link out the other side. Stop twisting just before the chain link pin is completely pushed out. If you push it all the way out, you won't be able to reconnect the chain links. When you begin to feel resistance on the handle as you twist it, stop twisting.
Remove the chain from the tool and twist it where you've pushed the pin. The links will separate as you twist. Twist one side of the link clockwise and the other side counterclockwise until they separate.
Remove the chain from the sprockets of the bicycle or motorcycle. Make the necessary repairs and then replace the chain. Snap the links back together.
Place the chain link back on the chain breaker tool's outermost slot with the link pin facing in toward the tool's pin. Twist the handle of the tool clockwise to push the pin back through the link plates until it's evenly protruding on both of the outer sides of the chain link plates.
Back off the handle of the tool and move the tool so that the chain link sits on its innermost slot. Twist the handle of the tool until the tool's pin gently taps the chain link pin. This will loosen any compression that has caused the sides of the chain link to bind and get stiff.