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 HTTPS is being used in core_config_data database table fields, to reach this database table you will need to open database first:
Base URL: web/unsecure/base_url
Base Link URL: web/unsecure/base_link_url
Secure Base URL: web/secure/base_url
Secure Base Link URL: web/secure/base_link_url
After this open your Magento admin page, and insert such parameters at Stores → Configuration → General → Web:
Where yoursite.com is replaced with your domain name.
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! 😊