Configuration of SSL must be handled at the IIS level, not within Kentico. Please see the following Microsoft documentation:
The feature within Kentico that uses SSL is the Document option under Properties -> Security that specifies whether a page should redirect to SSL (to use https instead of http). You can set this up per document, or you can set this in your master page and use inheritance to apply the setting to all pages.
This subject is explained in further detail in the following Kentico documentation: