What is the correct answer of how you've been?

The term 'How have you been?' is a standard formality in meeting or greeting someone. You have to judge for yourself how to answer it based on how well you know the person asking; how sincere or not the question seems.

Standard formal answer: 'Fine, thank you.' (unless you are seriously ill or dead)

More casual answer: 'I'm fine. How are you?'

Answer to someone who knows you well enough to be aware of any hardships you have recently encountered: 'I'm better, thank you.'; 'We're still working on it.'; 'The support of friends helps a lot.'; etc.

Surprisingly often, the greeting 'How have you been?' is answered by 'How are you?'; neither party actually answering the question.