You can apply for L visa extension at the Exit & Enter Department in China, please show your plane tickets and hotel reservations, it is not hard to get the extension because your flight tickets can prove you need to stay in China for another 5 days, you should apply for L visa extension on the third or fourth day after entering China, please pay attention to the processing time of the Exit & Enter Department, once your get the new visa, your present 10 days L visa automatically expires.