As with the Fedora 10 to 11 upgrade, you can upgrade Fedora 11 to Fedora 12 using yum. I find this to be the easiest and most reliable way to upgrade a Fedora installation whether you use it as a server or desktop.
To reduce the total data downloaded, I'd recommend installing the yum-presto package first. It downloads delta RPM's and builds them on the fly, which allows you to upgrade packages without having to download the entire RPM's.
yum install yum-presto
Now, upgrade your current system to the latest packages and clean up yum's metadata:
yum upgrade yum clean all
Get the latest fedora-release package and install it (replace x86_64 with x86 if you're using a 32-bit system):
wget ftp://download.fedora.redhat.com/pub/fedora/linux/releases/12/Fedora/x86_64/os/Packages/fedora-release-*.noarch.rpm rpm -Uvh fedora-release-*.rpm
Now, upgrade your system to Fedora 12:
yum upgrade
For detailed documentation on the entire process, refer to Fedora using yum on the FedoraProject Wiki.
