Zaphenath-paneah was the name that Pharaoh gave to Joseph when he made him governor of Egypt. This takes place in Genesis 41:41 in The Bible, so you can read it for yourself how it all happened. It was common in Egypt when for example a pharaoh ascended the throne for his name to be changed. I believe this is why Joseph's name was changed.
Zaphenath-paneah is an Egyptian name given to Joseph by Pharaoh in the Old Testament. It means "God speaks and he lives." Joseph was given this name when he interpreted Pharaoh's dreams and became a high official in Egypt.