Extension of stay

I have applied for Visa and got the same but the duration of stay is only for 15 days, normally it is 30 days every time,as I am a regular Visitor to China for my Business from Last 6 Years, This time I think to see all the 3 phases of Canton Fair, My Journey dates are 13th. April for entering and 5th. May for returning, Please advice me what to do as I need a extension of 8-10 days？