Whether you're a weekend trail rider or hard-core motocross competitor, you're going to end up with worn out parts on your dirt bike. Even if you follow all the chain tips we've covered elsewhere, it will still stretch and wear.
Some of the stretching is physical deformation of the links, however most of it is not actual factual stretching, but wear. Most of the wear will be in the chain rollers. As they wear, each one will get a tiny bit of extra play in them. These tiny bits of play all add up to create what most of us call "stretch". As the play between each roller gets bigger, your dirt bike's chain will start to ride higher on the sprocket teeth. The result is rounded or hooked teeth on your sprocket. Under normal circumstances, it pays to replace the chain and sprockets at the same time. The additional cost is minimal and prevents a bad sprocket from wrecking a new chain, and vice-versa.
Bear in mind that some chain stretching is normal. That's why you can adjust the rear wheel to sit further back, taking up some of the lack. But a chain breaking at speed has the potential to ball up at the case and cause some expensive damage.
You can tell if the chain is too far gone by supporting the chain in the middle (between the sprockets) with one hand, while using the other hand to try and pull the chain away from the rear sprocket. If the tension is adjusted as tight as it can go, and you can still pull the chain a quarter-inch or more from the sprocket, it's worn out.
Fortunately, replacing the chain and sprockets on a dirt bike is one of the easier maintenance jobs you can do on your own. This blog already has an article on replacing sprockets, so I'll mostly just cover the chain itself.
Start by removing the front chain guard and the front sprocket keeper.
Slip the chain off the front sprocket.
Once the chain is off the front sprocket, it's an easy matter to remove the rear wheel and remove it, along with the loop of chain.
Grab your new chain and reassemble everything in reverse order.
Yup. It's that easy.