Comment 12 for bug 1625805

@ppisati - if you have a classic deb based filesystem that is Xenial based, add the ppa:ci-train-ppa-service/stable-phone-overlay (update/upgrade), then you can simply apt-get install unity8-desktop-session-mir. Once that is installed, you can toggle between unity8 & unity7 by clicking the ubuntu-logo icon just in the top-right corner of the password entry box on the greeter/lock-screen to your session.
here's a doc for any other questions.
https://docs.google.com/document/d/1Io3pQvzyBQIpZi2n_hfseNsVOo4LH-BjtNhmdWNijEc