CentOS 3 offers both yum and up2date to do updates of the operating system. You can use either or both of these to manage your updates.
Prior to using yum, you must import the RPM-GPG-KEY for CentOS-3 like this
rpm --import /usr/share/doc/centos-release-3/RPM-GPG-KEY-CentOS-3
For more Information, see these Release-Notes, which are also in the /NOTES subdirectory on the CentOS-3 cd-1 and dvd, provided {and copyrighted} by Red Hat®,Inc. The CentOS project redistributes these original works (in their unmodified form) as a reference for CentOS-3. CentOS-3 is built from publicly available, open source SRPMs.