I’m not sure if I followed the right steps, but this is how to upgrade the Fedora IOT Edition from version 28 to version 29 (currently Rawhide).
$ sudo ostree remote add --set=gpgkeypath=/etc/pki/rpm-gpg/RPM-GPG-KEY-fedora-29-primary fedora-iot-29 https://dl.fedoraproject.org/iot/repo/
$ sudo rpm-ostree rebase fedora-iot-29:fedora/29/aarch64/iot
$ sudo systemctl reboot
But right now, once you are on F29 there seems to be a bug preventing further rpm-ostree upgrades.