This article applies to Fedora Atomic Host (though there is also an experimental workstation-ostree). For Fedora Workstation and Server, see DNF system upgrade.
Fedora Atomic Host installations use the rpm-ostree deployment method, where the same packages used in Workstation/Server are assembled on the Fedora release engineering side and delivered as versioned atomic units, rather than the traditional yum/DNF method of client side assembly.
It's crucial to note that at the moment, Fedora uses separate OSTree repositories for each major release. This makes switching between versions more painful. For more information, see this ticket.
Upgrading from Fedora 26 Atomic Host to Fedora 27 Atomic Host:
First, be sure you have at least a 2 GB of free space in the root partition. If you don't then add more space with a command like:
lvm lvextend atomicos/root -L +2G -r
Next add the remote, then rebase:
ostree remote add --if-not-exists --set=gpgkeypath=/etc/pki/rpm-gpg/RPM-GPG-KEY-fedora-27-primary fedora-atomic-27 https://kojipkgs.fedoraproject.org/atomic/27 rpm-ostree rebase fedora-atomic-27:fedora/27/x86_64/atomic-host
Like any other rpm-ostree update, this is staged for the next reboot, so to finally apply the update:
systemctl reboot
That should be all!