F Visa extension with multiple entries

I have Pakistani Passport and PR card holder of Canada.I live Toronto.I got my one year F type for China from Toronto with multiple entries valid up to 07 January 2014. I want to go to China in first week of December 2013.Further I want to stay up to March 2014 due to my business meeting.
Now the present one will expire on 07th January 2014.For my further stay can I get my F type extended for valid entries from Toronto or I apply for extension in China while reach there.What will be better option and can any one suggest?