It helps to understand that asking how someone is at the start of a conversation should generally be interpreted as a social convention rather than a literal question.

The socially correct response is either something like "fine", or to repeat the question. Note that if you're going to answer it as a question, the correct answer is always "fine" regardless of whether one really is fine; if you said something like "terrible" you'd be obliging a stranger to pry into your personal business. The fabric of society is very complex, as someone once said.

These are all valid in various dialects/registers of native English:

Greeting: How are you?

Response: Fine, thank you

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Greeting: How are you?

Response: How are you?

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Greeting: How do you do

Response: How do you do

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Greeting: Sup

Response: Sup