Add this line into /etc/nginx/nginx.conf:
... server_tokens off ...
where this line of config will hide your nginx version number.
/etc/init.t/nginx restart
To check for the header, you can use curl:
curl -I www.foo.net
where -I is for curl to grab just the http header.
returns:
HTTP/1.1 200 OK Content-Type: text/html; charset=utf-8 Server: Apache/2.2.15 (Red Hat)