Learn German: Um ganz Asien zu verstehen, muss man ganz China kennen. - In order to understand all of Asia, one must know all of China.

This is a special exception in German. When ganz (whole/all of) or halb (half of) comes directly before a geographical name that doesn't use an article (like most continents, countries, and cities), it takes no adjective ending. If the country did require an article, like die Schweiz, you would decline the adjective normally: die ganze Schweiz.

Why does the second part of the sentence start with the verb muss rather than the subject man?

The entire first clause (Um ganz Asien zu verstehen) occupies the first position of the overall sentence. Because German strictly requires the conjugated verb in the second position, muss must come immediately after the comma, which pushes the subject man into the third position.

Could I use wissen instead of kennen to say "know all of China"?

No, you must use kennen here. Kennen means to be familiar or acquainted with a noun (a person, place, or thing). Wissen is used for knowing facts, knowing that something is true, or knowing how to do something. Since China is a place you are becoming familiar with, kennen is the correct choice.

What is the function of the um ... zu structure at the beginning?