Mixed content occurs when you attempt to load images or other content using HTTP instead of HTTPS. To fix this, all you need to do is edit the links on your page so that they start with HTTPS instead.
To force your Joomla website to use HTTPS, navigate to Global Configuration → Server settings, locate the Force HTTPS option, and choose Entire site:
Next, open your website's .htaccess file using the file manager and add the following lines after RewriteEngine On:
RewriteCond %{HTTPS} !=on
RewriteRule ^ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]Save the changes and reload your site - it should be working properly with HTTPS.
If you still get a mixed content warning, add the DB Replacer extension to your Joomla installation and use it to change all HTTP links to HTTPS automatically.
Learn more: Enabling HTTPS on your site - Joomla! Documentation 💡