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.

Now, upgrade your current system to the latest packages and clean up yum’s metadata:

Get the latest fedora-release package and install it (replace x86_64 with x86 if you’re using a 32-bit system):

Now, upgrade your system to Fedora 12:

For detailed documentation on the entire process, refer to Fedora using yum on the FedoraProject Wiki.