Changing HTTP TO HTTPS- failed to open page

Review these resources and see where you made a mistake:
https://make.ww.wp.xz.cn/support/user-manual/web-publishing/
https-for-wordpress/ http://www.wpbeginner.com/wp-tutorials/how-to-add-ssl-and-https-in-wordpress/

Also check out this plugin: https://ww.wp.xz.cn/plugins/really-simple-ssl/

Did you verify that your https redirect work properly? This tool is great https://httpstatus.io

You may access your server or files via FTP/ SFTP , or a file manager in your hosting account’s control panel (consult your hosting provider’s documentation for specifics on these)