There are a lot of different ways to answer when a stranger says “what’s up?” Often it’s just another way of saying “hello.” So you could simply respond with another greeting, depending on what you’re comfortable with (hello, hi, good morning, etc). Sometimes it’s a way of asking what you’re doing, but in a friendly, curious way. You can respond however you’re comfortable (for example, “just trying to finish this project” or “I’m headed to the grocery store”). If someone looks concerned when they ask “what’s up,” they’re asking what’s wrong and if you’re ok. Again, respond with what you’re comfortable with. If it’s a stranger you would like to keep interacting with, you can respond with things that invite further conversation. But if it’s someone you want to go away, you can close the conversation with a non-engaging answer such as “I’m working” or “I’m fine, thank you.” You will get different reactions depending on where you are (especially different regions in the United States) but as long as you are polite you likely will not have much problem.