When in China you can exchange currency in most banks and the Post office. You may find a better exchange rate before you travel so it is best to shop around.
Finding a bank is easy, in Chinese cities you will find a bank every 5-10 minute walk. Many ATM’s will also allow you to take funds directly from your home account but they will likely charge you a fee.
Never exchange your money on the street or in small shops. Avoid the Chinese “black market” for exchanging foreign currency as you may get ripped off with counterfeit bills.