Is grammatically correct to say how fun.?

"How fun" is not grammatically correct on its own. It needs to be part of a complete sentence or question.

Here are some correct ways to use "how fun":

* "How fun is that?" (Asking a question about something)

* "That was how fun it was." (Using it as a predicate nominative)

* "Can you tell me how fun it was?" (Using it in a question)

Essentially, you need to provide context for "how fun" to make sense.