Breaking the chain
Remove the master link with a flat head screw driver and count the links on the old chain. Don't take off more links than you need to. This is a step that cannot be undone easily, so make sure you know exactly how many links you are removing.
Line the chain breaker up with the rear pin of the link you wish to remove. If you do not have a chain breaker, you mush file or grind the head off the pin until it is flush with the link.
Twist the chain breaker screw bolt until there is tension on the pin and, using an adjustable wrench, twist the chain breaker screw bolt until the pin is pushed out the bottom of the chain. If you are using a grinder, once the head of the pin is ground down, place the punch on the head of the pin and hammer the pin out through the bottom of the chain. Make sure your punch is a smaller diameter than the pin so it can easily pass through the chain. Also make sure that there is an open surface below the link or you will be pounding the link into the hard surface below.
Reinstall the chain onto the ATV, checking both fit length and path of the chain. Make sure the chain is on the sprockets and rollers correctly.
Use the master link to reattach the chain to itself. With a flat head screw driver spread the link clip apart slightly to slip it over the master link pin while holding it in place with a pair of pliers. This can be a tricky task but some patience and concentration will go a long way.