I had applied for a Schengen visa with French embassy this March 2015, and had a visa from April-September 2015. Now I want to visit Switzerland in Dec 2015. Should I now apply for a new visa with Swiss embassy or is there an option to extend/renew my old Schengen visa?
If your existing Schengen visa is expired, you must apply for a new one.
Because Switzerland is your "main destination", you must apply to the Swiss authorities.
Quoting the French embassy on the matter:
Previous grants of visas do no automatically qualify you for a renewal of the same or longer visa. A visa is never renewed or extended, each application is a new application. Your personal situation will be studied by the immigration officer each time you apply and the consular administration will determine if you qualify for a visa.