The recommended installation method is detailed in the Installation Guide:
The release notes also have some useful information (the following link is for the Fedora 21 release notes):
PreUpgrade is an application users run on an existing Fedora 7 or above installation, that resolves and downloads packages required to upgrade Fedora. While PreUpgrade downloads the necessary packages, users are free to continue using their systems.
- It's a good idea to have a backup of your system before performing an upgrade. If you have
/homein a separate logical volume or partition, it makes backing up user data easier. This is a feature requested for the Fedora Installer. See Bug 150670 for more on this.
- Doing a clean installation and then restoring user data from backups may work better for some users. Future releases may include features to assist in this process. See AnacondaWorkItems for more information.
- You can do an upgrade using the regular installation DVDs. Live media only performs fresh installations, not upgrades. The installation overrides any third party packages which conflict with the default installation set. Applications within the Fedora repository are easily upgradeable. See the Installation Guide for more information.
- Make sure you read the Release Notes carefully before attempting an upgrade.
- You can also do an upgrade using the
yumpackage manager, but this is not officially supported by the Fedora Project. See the Yum Upgrade FAQ page for details.