It has generally been difficult to test new updates for the rpm-ostree or ostree packages for Atomic Host. This is because in the past you had to build your own tree in order to test them. Now, however, Fedora has starting building a tree based off the updates-testing yum repositories. This means that you can easily test updates by simply running Fedora Atomic Host and rebasing to the fedora-atomic/f22/x86_64/testing/docker-host ref:
# rpm-ostree rebase fedora-atomic:fedora-atomic/f22/x86_64/testing/docker-host # reboot
After reboot you are now (hopefully) booted into the tree with updates baked in. You can do your tests and report your results back upstream. If you ever want to go back to following the stable branch then you can do that by running:
# rpm-ostree rebase fedora-atomic:fedora-atomic/f22/x86_64/docker-host # reboot
Testing updates this way can apply to any of the packages within Atomic Host. Since Atomic Host has a small footprint the package you want to test might not be included, but if it is then this is a great way to test things out.