How do you reply back to 'muy bien' in spanish?

Buenos días means "good mornings." It is a plural, normally translated in the singular as "Good morning." You could also say "buen día" and still be correct, but maybe a little abrupt.

In response, you would normally repeat the statement. "Buenos días", and then make a polite inquiry. "¿Cómo está usted?" or "¿Cómo estás?" If you are familiar, you might say "¿Qué tal?" or "¿Qué pasa?"