To hide the detailed information about your Apache web server, OS and plugins, you disable the ServerSignature directive by modifying your Apache configuration file:
On CentOS
vi /etc/httpd/conf/httpd.conf
On Ubuntu
vi /etc/apache2/apache2.conf
Search for ServerSignature and change it to off
ServerSignature Off
Save the config and reload Apache to apply the change:
On CentOS
service httpd reload
On Ubuntu
/etc/init.d/apache2 reload