What is an antonym for how?

There isn't a true antonym for "how" in the way that "hot" and "cold" are antonyms. "How" is an interrogative adverb asking for the manner or method of something. It doesn't have a direct opposite in meaning.

However, depending on the context, you could use words like:

* Why: If you want to know the reason for something.

* What: If you want to know the thing itself.

* When: If you want to know the time.

* Where: If you want to know the location.

For example:

* How did you get here? (Asking for the method of travel)

* Why did you come here? (Asking for the reason)

* What brought you here? (Asking for the thing that caused the travel)

Ultimately, the best alternative to "how" depends on what specific information you are trying to elicit.