https://wordpress.org/plugins/wp-force-https/
若只需后台强制https,前台仍为http,则有:
https://github.com/phikai/wordpress-force-http
用下面的插件把原http引用的图片和静态资源全部改成https,
https://wordpress.org/plugins/http-https-remover/
并将页面中手动植入的其他http全改成https。