How can I extend my L visa in PRC?

Hi,I'm coming to china on 5 /1 /2014. I have got L type for 20 days, so I want to register in many universities and I think 20 days is not enough, so could you help plz ? I wanna now how I can renew my L type? And where I can ( in which city ) ? Do I have to go to specific city? Before how many days I have to apply for it? After how many days they will give me respond? Thanks