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!"
I tell folks "I'll do my best." then often people remark ,"that is all we can do." I say "no, I have done much less before." "best is a lot of work." they usually agree.
Warren Zevon sang Looking For The Next Best Thing about exactly that. "I appreciate the best/but I'm settling for less/I'm looking for the next best thing."