Extending Duration of Stay

I'm currently in Beijing on a double entry F-visa. The visa is split into two 90 day stays, the first of which will expire at the end of the month. Is it possible to extend this stay or transfer to different visa as im only staying for another 35 days or so. Plus the internship I am doing comes to an end at the end of the month. It would be a big help as it would save a costly trip to HK or a long one to Mongolia.