How can i downgrade to CS5.5 from CS6 windows license?
You can't. If you never bought CS5.5 there is no way for your to obtain a license at this point.
Mylenium is mostly correct. If you have a volume license and others have CS 5.5 then you can downgrade. It requires you to contact your reseller. However, for normal mortals, you cannot downgrade to CS 5.5.