I'm holding S2 visa for treatment in Beijing

FIRST, thanks for your help.
I'm holding S2 type for treatment in Beijing for 3 months. However, the doctor asks me to stay here for another 3 - 4 months depending on results of examinations and treatment. I want to ask if I can extend my S2 for another 4 months. Is this possible to extend for more than the original duration? what should I do? another question, should I go to any PSB in beijing or a specific one?