https site shows "Show all content" in all browsers

Hi Team,

 

We are developing asp.net website.Frame work : 3.5 ,visual studio 2010.

Recently website is changed http to https. We are displaying external websites in this "https"  site.Then it shows "Show All Content " in All browsers.

If we click on "Show All" browser message then the site will refresh and again click on the external "http" site link  display in "https" site.

 

This message is frustrated to Customers.So how to display external sites"http"  in "https" without displaying these type of messages?

 

Thanks.