Have a blessed day . I have no idea how to respond anyone???
If it's a stranger, I just smile to myself and put a puzzled look on my face and go on my way without a response.
If it's an acquaintance and they don't know me well enough to realize that's not something I'd enjoy hearing, they don't need a response of the same words. I might instead say "I'm having a great day, hope you are too!"