Presently, Ulefone Vienna users can only install the beta Marshmallow via USB flash while OTA is not enabled in the ROM, so when the final version comes, they will have to flash again. If you don’t want to stay on the trial ROM, you can still flash back to Android 5.1 and wait for the OTA upgrade. Besides the features born with Android 6.0, ulefone will also add some software tricks such as multitask window, graphics interface in Settings app, and manual mode in camera app to the final firmware as they announced before.
Ulefone formerly said they’ll upgrade the Vienna to Android 6.0 before the end of June while some delay is expectable. Since the beta version has come, it is hopeful that the delay will not be that long.