If you will stay in China for a short time (e.g. one to two months), you do not need to do anything and just get a new visa to come back and get a new I-94. If you will stay in China for a longer time (e.g. > 3 months), you need to send the I-94, the airplane boarding card, the passport page showing the Chinese Customs stamp which indicates you entered China on what date and a explanation letter together to the following address. Not need Fedex, and regular mail is OK. Keep the photocopies of all the documents in your file in case:
ACS-INS SBU
P.O.Box 7125
London, KY 40472-7125
