-Have you stayed in China continuously for more than one year without reentry permit? If yes, your green card is invalid and you can not renew it. If no, you need to get the transportation letter in any one of the US Consulates in China to come to US, and then you can file I-90 to get the renewal of green card.
See:
http://guangzhou.usembassy-china.org.cn/uscis-transportation-letters.html
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