I think that the modification you want to make is really just an act of sleight of hand -
anyone that really wants to know can very easily determine your versions of apache, php and openssl. Additionally, there are a number of fingerprinting applications that can provide them with your OS and OS version, too.
That said,
here is a tutorial that may help you accomplish this.
Really, though, active protection like mod_security and phpIDS might do more to protect you than simply not displaying those version numbers.