If you are getting this popup warning, then it means you have used http instead of https, removing this warning very simple, you just need to replace the http with https.
For example: If you find a link like this:
<a href= "http://www.yourdomainname.com/about/index.php">Click Me</a> change it to
<a href= "https://www.yourdomainname.com/about/index.php">Click Me</a>
The same for images, change <img src="http://www.yourdomainname.com/images/logo.gif"> to