Renewing a 1 year multiple entry F visa.

I am a US Citizen and currently in China on a 1 year multiple entry F. I need to renew it in August and wanted to know if it's possible to renew it for another year without leaving? If I do have to exit, can I get it renewed in Hong Kong? I thought I had read that F renewals are granted for only 6 months. Is this true? Would I need to go back to the US to get it renewed? What type of documentation will be required for this process. Thank you!