I'm a British citizen that lives in New York (British passport and US greencard).

I'm planning the following trip in March:
NYC to Hong Kong (5 days)
Hong Kong to Shanghai (5 days)
Shanghai to NYC (via Hong Kong)

I gather from searching the forums that I don't need a visa to enter Hong Kong, but I can't seem to clarify if I need a visa to travel from Hong Kong to Shanghai?

If I do need a visa can I obtain this in NY for my UK passport?
Is it possible to wait til I am in Hong Kong and potentially obtain one there?

Many thanks in advance for any advice!