I would like to know what to do with the zip file after down loaded.
I did unzip the Core (files) to C:\Programfiles(x86)\Arduino\Hardware\Arduino .. but when I open the Arduino ide the STM option is not available in Board Manager.
Where did you get your zip file from? I know at least 2 different cores for STM32F1.
Usually it is the best to get a .json install path, copy it as additional Boards Manager URL in preferences and install the core by means of the boards manager. If there are already other URL's append it separated by a comma.