L VISA EXTENTION

I have read all the questions and answers provided to all those questions.I hold a turkish passport and came with a L type with my family this month the 8th of Feb,'2014,presently in Tianjin,l was issued double entry, which l think expires in April. Well,l want to extend that L type (TURKISH PASSPORT HOLDER ),or if so l would not mind to convert it to Z type because l was offered a teaching position in Tianjin. pls your advice will be highly regarded.