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.
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:
That should be all!