Mixed content means that you are trying to load images or other content with HTTP instead of HTTPS in their lins. To fix this, you would simply need to edit all the links on your page to have HTTPS at the beginning instead.
First, force your PrestaShop website to use HTTPS by navigating to Configure → Shop Parameters → General. Just choose Enable SSL on all pages:
If you still see Mixed content on your website, please go to Preferences → CMS and edit images src to HTTPS instead.
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! 😊