CONCLUSION
The final step in the scientific method is the conclusion. This is a summary of the experiment's results, and how those results match up to your hypothesis.
You have two options for your conclusions: based on your results, either you can reject the hypothesis, or you can not reject the hypothesis. This is an important point. You can not PROVE the hypothesis with a single experiment, because there is a chance that you made an error somewhere along the way. What you can say is that your results SUPPORT the original hypothesis.
If your original hypothesis didn't match up with the final results of your experiment, don't change the hypothesis. Instead, try to explain what might have been wrong with your original hypothesis. What information did you not have originally that caused you to be wrong in your prediction? What are the reasons that the hypothesis and experimental results didn't match up?
Remember, a science fair experiment isn't a failure if it proves your hypothesis wrong or if your prediction isn't accurate. No one will take points off for that. A science fair experiment is only a failure if its design is flawed. A flawed experiment is one that (1) doesn't keep its variables under control, and (2) doesn't sufficiently answer the question that you asked of it.