It depends on the model selected. Newer unlocked devices like the NUU X3 are built on the Android™ KitKat 4.4 operating system and will be upgradeable to Lollipop 5.0, while older devices may not be updateable.
NUU uses the most current Android™ OS when manufacturing their devices; phones are currently being manufactured with Android™ 5.0 Lollipop.