Android Pie rollout season is currently completed its first phase in the US and the pie has reached to Samsung's previously released flagships. Meanwhile, US unlocked Galaxy S9 and S9+ are receiving march security patch.
This update for unlocked Galaxy S9 devices brings March 2019 that provides fixes to 11 new Samsung Vulnerabilities and Exposures (SVE) found in the recent software version.
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The build version for this update is - G960U1UEU4CSC7 and G965U1UEU4CSC7 for Galaxy S9 and S9+ respectively. If you want to check the update manually please go to the Settings > Software update > Download and install.