Using an FTP tool, add the following content to .htaccess.
RewriteEngine On
RewriteCond %{HTTPS} !=on
RewriteRule ^ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]To prevent errors, add the following content as well.
RewriteOptions inherit
RewriteEngine on
Header set content-Security-Policy: upgrade-insecure-requests