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 HTTPS on your PrestaShop website, go to your PrestaShop administration panel, navigate to Configure → Shop Parameters → General, and choose Enable SSL on all pages:
Now, all the pages on your website will load using HTTPS. You can verify it by checking your website on an incognito window.
If the mixed content issue persists, open your website's .htaccess file using the file manager (or create it if non-existent) and insert the following line of code:
Header always set Content-Security-Policy: upgrade-insecure-requests
Save the changes and reload your site - it should be working properly with HTTPS.
If your website uses symlinks, applying the above-mentioned change to the .htaccess file is not recommended 💡