"What" is a describes the item itself, and "how" is the method.
In such a short sentence, the rules are clear: "What do you say it" is incorrect, because you cannot "say" a description.
You can say, "How do you say it" or "How do you call it", as it refers to the method of doing something. The answer could be the method or the name of the item (e.g. "The answer is a car" or "I call it a car")
"What do you call it" is almost the same, but generally it is to describe what the thing is (e.g. "It is a car"). If I answer "The answer is a car", it would sound unnatural.