I am in China now with a single-entry visa but decided to go back next weekend for a short trip.

I'm in China for an internship until end of Feb and I'm planning to go back to Malaysia during the CNY holidays. Because it's a single entry, I still need to apply/extend it so I can come back to continue my work. I'm wondering if you know how I can extend/increase the entry while I am still in Shanghai? And where to do it? Also, how long would that take roughly?
Many thanks!