This is how I updated my FC6 to F7:
rpm -U fedora-release-7-3.noarch.rpm fedora-release-notes-7.0.0-1.noarch.rpm
In theory that’s the only thing you need to do, in practice it’s not that easy. I had some dependency issues with pidgin and liferea so I just removed them:
rpm -e pidgin liferea
Then yum complained about libexif being “not signed”. I tried to edit /etc/yum.conf and setting gpgcheck=0, that didn’t work, yum was still checking it. So I had to manually set gpgcheck=0 in all the repos on /etc/yum.repos.d/. Then it worked.