You can add this to your .htaccess file to help with the problem, but there is still work to be done, like changing all the URL in the video plugin etc.
# Force SSL:
RewriteCond %{HTTPS} offRewriteRule (.*) https://%{HTTP_HOST}%{REQUEST_URI} [R=301,L]
<IfModule mod_headers.c>
Header always set Content-Security-Policy "upgrade-insecure-requests;"
</IfModule>
Or you can do it in the custom header code box.
<meta http-equiv="Content-Security-Policy" content="upgrade-insecure-requests">