1. Upgrade the fedora-release packages, e.g., 
        rpm -Uvh \
      http://<mirrorsite>/fedora.redhat.com/fedora/linux/core/6/i386/os/Fedora/RPMS/fedora-release-notes-6-3.noarch.rpm \
      http://<mirrorsite>/pub/fedora.redhat.com/fedora/linux/core/6/i386/os/Fedora/RPMS/fedora-release-6-4.noarch.rpm
    Note that if you have modified the fedora files in /etc/yum.repos.d, the above RPM command will write some files with suffix .rpmnew — overwrite the contents of the corresponding files with the contents of these new files.
  2. Clear out old (cached) package data: 
            yum clean all
  3. Then 
            yum update yum
  4. Then 
            yum update
Going from FC5 to FC6 on PPC64 I found many problems which required manual intervention, but it worked in the end.