CentOS has released ver 5.9 for i386 and x86_64 architectures. CentOS 5.9 is based on the upstream release Enterprise Linux 5.9 and includes packages from all variants including Server and Client. All upstream repositories have been combined into one, to make it easier for end users to work with.
You can download CentOS using a bittorrent client. CD torrents [32-bit, 64-bit], and DVD torrents [32-bit, 64-bit].
Alternately, you can directly download the ISO images from - isoredirect.centos.org/centos/5/isos/.
If you are running an earlier version of 5.x CentOS release, you can upgrade very easily via the command line.
To check the version of CentOS you are running, do -
rpm -q centos-release
You can upgrade from an older CentOS 5.x release in a simple manner -
# yum update
If you are running CentOS-5.8 and have the CR repo (short for 'Continuous Release repository') enabled, a simple 'yum update' will still move your machine to 5.9.
A CR repo is similar to a rolling release system where your OS keeps on getting continuous upgrades.
To get more help with CentOS refer - wiki.centos.org/GettingHelp and CentOS FAQ.
Also read - CentOS 5.9 full release notes.