So, I have RedHat 9.0 in my server and no previous experience on installing Apache or PHP and I'd like to update Apache 2.0.40 to .47 and PHP to 4.3.2.

Where should I start, detailed instructions?

Download Apache from apache.org, and php from php.net
Read the INSTALL and/or README files for instructions on how to compile the sources
I haven't built the php module for apache yet, but I think that basically when you do ./configure for apache, you need to add

--enable-module=php

or whatever the name of the php module is.

Do I have to remove the previous installation? If yes, how?

You don't need to touch the old installation (i assume it came with RH9) Your new apache 2 would install, by default, in /usr/local/apache2. But to start it, you have to manually execute apachectl from /usr/local/apache2/bin, since the $PATH variable, which defines where to search for the command apachectl, points to the directory /usr/sbin, or something similar.

And if you want the new apache to be started at bootup, you need to edit a bootup script, /etc/rc.d/init.d/httpd

There are two definitions there:

apachectl=/usr/sbin/apachectl
httpd=${HTTPD-/usr/sbin/httpd}

you simply have to change that to the new installation, and it'll work. Have fun