WordPress is a free and open-source content management system written in PHP and paired with a MySQL or MariaDB database. Features include a plugin architecture...
Check the browser’s exact warning. An expired certificate or one that does not cover the address you are visiting is different from mixed content: an HTTPS page loading images, scripts or other resources over HTTP. Your browser’s developer tools can help identify those resources.
Check that the certificate is valid for the address in use and that the flagged resources are available over HTTPS. In WordPress, also check the site addresses in the settings. Save a database copy before making bulk replacements, and use a method compatible with WordPress data. Afterwards, clear the cache and test your key pages.