Mixed content means that you are trying to load images or other content with HTTP instead of HTTPS in their links. To fix this, you would simply need to edit all the links on your page to have HTTPS at the beginning instead.
In Joomla, it can be done automatically. First, force your Joomla website to use HTTPS by navigating to Global Configuration → Server settings, and in Force HTTPS choose Entire site:
Then open your website files through File Manager. Add these lines to your .htaccess file after RewriteEngine On:
RewriteCond %{HTTPS} !=on
RewriteRule ^ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]
Check if configuration.php file has the full website URL:
var $live_site = 'http://www.my-domain.com';
If yes, change the protocol from http:// to https://
If you still see that you have some Mixed content, you can go ahead and install DB Replacer extension to your Joomla installation and change all HTTP links to HTTPS ones to save you from the manual work of changing each and every link manually.
That's all! You are good to go, and shall have your website displayed as Secure now 😊