To redirect all HTTP requests to HTTPS, add a new nginx server block that looks like this:

server {
    listen 80;
    server_name www.yourdomain.com yourdomain.com;

    return 301 https://yourdomain.com$request_uri;
}

In order for that to work properly, your other server bock that does the reverse proxying must only listen on port 443/SSL. For example:

server {
    listen 443 ssl;
    server_name www.yourdomain.com yourdomain.com;
    ssl_certificate ...
}

Finally, reload nginx after making the changes to the config files:

sudo service nginx reload