Add these lines into /etc/apache2/apache2.conf:

/etc/apache2/apache2.conf

ServerTokens Prod
ServerSignature Off

where ServerTokens controls what kind of information is sent to the header. Options available for ServerTokens include:

ServerTokens Prod[uctOnly]
Server sends (e.g.): Server: Apache

ServerTokens Major
Server sends (e.g.): Server: Apache/2

ServerTokens Minor
Server sends (e.g.): Server: Apache/2.0

ServerTokens Min[imal]
Server sends (e.g.): Server: Apache/2.0.41

ServerTokens OS
Server sends (e.g.): Server: Apache/2.0.41 (Unix)

ServerTokens Full (or not specified)
Server sends (e.g.): Server: Apache/2.0.41 (Unix) PHP/4.2.2 MyMod/1.2

while ServerSignature is to allow configuration of a trailing footer line under server generated documents, such as error messages and mod_proxy ftp directory listings. Putting it to off will suppress the footer line.

Once added, restart or reload apache to activate the changes.

/etc/init.d/apache2 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)