I think the most easiest way to upgrade Fedora 11 to Fedora 12 is program called preupgrade, which download needed packages from the server and then just reboot to installer and after install boot the new system. Of course it’s important backup your important files before upgrading.
Commands and screenshots of whole upgrading process
First start with updating all Fedora 11 packages with following command as root:
Install preupgrade as root:
yum install preupgrade
Start preupgrade as root:
Preupgrade information window
Choose desired release
Preparing system for upgrade
Prepare and test upgrade
When ready to begin upgrade then reboot.
Starting Fedora 12 installation
Installing Fedora 12 packages
Booting just upgraded Fedora 12 system
Fedora 12 desktop
Note: If you have any problems with Xorg when booting the Fedora 12 then it’s good read this.