How do Tom and Huck confuse the Phelps? What is their purpose in doing this.

They think that Huck is Tom (their nephew), and that Tom is someone else. They don't realize the boys have changed places.

But since you've mentioned so many chapters here, you might be talking about the explosion, and how Huck convinces everyone that a "slave" was killed. Therefore the Phelps are taken off the scent of Jim's escape.