There is no set way to reply to a birthday greeting; different situations have different ways in which it is appropriate to respond. For example, a child receiving a birthday card from a school...
In Mandarin "Nin2 hao3" is a formal greeting and "Ni3 hao3" is an informal greeting. I'm not quite sure what the Cantonese formal greeting is, but "Nei hou" is the informal greeting. If you are...