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.
First, make sure that both config.php files (one in your root public_html directory and one in /admin/config.php directory) have HTTPS URLs leading to your domain. In order to replace links easily open the configuration file via File Manager and press Ctrl + H. We need to replace http:// with https:// as shown below:
And then in your OpenCart admin dashboard open Settings → Server. In Security section choose Use SSL as Yes:
Now your website shall be working with HTTPS 😊
If you still see that you have some Mixed content on the website, you can add this line to your .htaccess file:
Header always set Content-Security-Policy: upgrade-insecure-requests
It will also force all mixed content to use HTTPS.
That's all! You are good to go and shall have your website displayed as Secure now! 😊