Extension of double entry visa

Hello, we have a double entry tourist L with a duration of 60 days. We won't be able to leave and reenter because it wont be valid anymore (our visa was valid until the 12th of september, we entered on the 2nd of september for the first time). Is it possible to get an extension for my current one? If so, is there anywhere where we find can find a list in which cities we can find PSB to do so?
Thank you very much for your information!