We recommend everyone run through a reinstall rather than attempt an
inplace upgrade from CentOS-4 or CentOS-5!
Source: http://lists.centos.org/pipermail/centos-announce/2011-July/017645.html
You can run this upgrade centos6 just by using yum upgrade --enablerepo=centosplus.
Do not forget: each system running centos is individual (!). I recommend you to test this upgrade on a virtual machine before upgrading (i did so!). Of course, this process may takes some hours. I’ve upgraded several internal systems virtually today and monitoring them right now. regards, msec.