Extension of F visa from 60 days to 180 days or more

I have a 2-year, multi-entry F visa with 60 days duration. I have a customer in Xi'an that wants me to stay longer because the technical consulting I am providing is needed for a longer period of time. When I am not working I stay with my Chinese wife in Nanning, Guangxi province. How should Iproceed to obtain an extension for 180 days or longer?