Tracking users switch to the new version of the app [on hold]

The web-app I'm working on is going to be released soon offering the soft-transmission for current users, they will be able to try new UI, and revert back to the current one. The task I'm trying to complete is to set-up the Google Analytics to track ratio of the users who clicked (1) 'Try new app' and whether they are switching back (2). How to do it properly?