Typically a polite response will suffice. Consider asking them a question in return. Most people like to talk about themselves. So you could say, “I’m fine, and you? How’s your son doing in basketball these days?” or something.
You can also just re-direct the conversation. If there’s something you don’t want to talk about just change the subject. So if they are asking about your health, you can say, “I’m feeling good these days. I’ve started walking daily and going to the gym.” or “I’m feeling good these days. Been spending a lot of time watching my grandkids! They keep me busy!” That way you shift the discussion from your health to your exercise, or from your health to your grandkids.
FWIW, although I’m fairly scrupulous about lying, I don’t think there is anything wrong with saying you’re “fine” even when you really aren’t but if it bothers you you might say something like, “Getting by” instead.