I have tested this method, but if you have problems, please let me know.
1. Before Upgrade
1.1 Backup Your Data
Before doing anything, do full system backup or backup at least your important data: documents, pictures, videos, config files, log files, databases etc. This is very important, because if something go wrong and you have to do fresh install, then you can easily restore your data.
2. Upgrade Fedora 17 to Fedora 18
2.1 Change root user
su - ## OR ## sudo -i
2.2 Update Fedora 17 Packages
2.3 Reboot Your System
Just make sure that you are running latest kernel.
2.4 Install FedUp
It’s probably installed already.
yum install fedup
2.4 Start Upgrading Using FedUp
fedup-cli --network 18 --debuglog /root/fedup.debug.log
When this finish, check if there is any errors in fedup.debug.log.
You can enable additional repos, with using following option:
2.5 Reboot System