How to upgrade from F17 to F18 (courtesy of Jon Masters):
Install the rpm by downloading both of the noarch packages there.
Install them with rpm, or with "yum install fedora-release*.rpm"
Then do a "yum install rpm yum" to upgrade those bits.
You will see some failures during that process due to the linker path changing and the upgrade not working (we don't really do upgrades per-se on ARM devices at the moment).
Shutdown the system with a halt/poweroff/or powerbutton press/hold.
Take out the SD Card. Insert into another system and make a symlink from /lib/ld-linux.so.3->/lib/ld-2.16.so (which should happen during a glibc upgrade but actually isn't being created properly right now).