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, force your PrestaShop website to use HTTPS by navigating to Configure → Shop Parameters → General and choosing Enable SSL on all pages:
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 the mixed content to use HTTPS.
That's all! You are good to go and shall have your website displayed as Secure now! 😊