Some more information:

I copy the following from the website US Consulate in Guangzhou, China at http://guangzhou.usembassy-china.org.cn/iv_faqs.html#B14


14. I need to change my appointment date. What should I do?

You may request to change or postpone your interview date by contacting the Consulate either by mail, fax, or email. Be sure to include your name, case number, and your updated mailing address in Chinese characters. Also include a contact phone number where you can be reached.

Your new interview date will be set according to the Consulate’s interview schedule. Please note that in periods of high demand, this may up to several months. The Consulate will send you a new appointment package as soon as your next interview date has been set.