In conversation, it is understandable. In writting, the first letter in a sentence must be in capital letter, period.
By concept, " how have you been?" is only a compliment, there is no special answer for it. Any answers are correct answer even it is out of the question.
How have you been? (you may raise your voice to make a cunter question)
Still surviving, how about you?
All answers given by others are correct. My answer is a suppliment to their answers, it is not traditional, you may not found them in books.