Customizer using only http

How do I get the Customizer to use https instead of http?

I recently switched to SSL and I am getting mixed content because of my header image. I open Appearance > Header. I select Add Image on the current header and it takes me to a screen that says Choose Image and shows me pictures in my media library. I select my current image, or any other image, and the right side of screen shows attachment details. The url shown for the image is https:// and I cannot change it. I would like to change it to https://, but some sites have recommended the use of //, so it works with whatever protocol the site is using. I know I do not want it to be http, so how do I change that url? I presume changing/fixing that will fix my mixed content problem.