If the belt is worn out it doesn't means you need to replace the treadmill, belts can be replaced just like other components of the item can. First you need to evaluate if your treadmill belt is worn out. What you need to do is run it at a very slow speed without you getting on it and look at the edges to see if they've curled up or are frayed.
Next have a look at the seam to see if any part of it is ripped or frayed too. This is a vulnerable area of the belt so problems are likely to occur here first so have a good look. Turn the treadmill off and unplug it and lift the belt up with your hands and there should be 3 to 4 inches of give. If there's more then tighten the belt using the screws at the end. Keep an eye on this to ensure the belt hasn't lost it's rigidity.
If it does need to be replaced then check to see if your treadmill is under warranty. If so talk to customer service at the manufacturer and organize the repair. If not they still may be able to help you or choose a repair man of your own. Don't try to fit the new belt yourself if you're not experienced enough. It may void the warranty and you may damage the treadmill.