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
