The most recent version of the Anaconda, starting with Fedora 18, does not provide any means for upgrading from one release of Fedora to another. See my review of Fedora 18 for details.
To overcome this lack of an upgrade procedure, new upgrade software named fedup has been developed since the introduction of the completely rewritten Anaconda installer.
There are many bugs in fedup. It is, after all, a 0.8 version. One bug that indicates problems upgrading GPG keys can be overcome using the following command.
rpm --import /etc/pki/rpm-gpg/RPM-GPG-KEY-rpmfusion-nonfree-fedora-2
Other extant bugs are a bit more intractable. Many folks, including myself, seem to be unable to overcome them, although some users seem to have no trouble using fedup for upgrades. I have tried to use fedup to upgrade from Fedora 18 to 19, and now from 19 to 20. Even in a simple virtual test machine it fails to actually perform the upgrade. This is not very promising for use on a production host.
So for the moment, I am fed up with fedup. I recommend against using it on production systems unless you have thoroughly tested and are satisfied that the upgrade can be performed without issues.