Joining the league with Galaxy Note 9 and Note 8 devices, the Galaxy S9 and S9+ are also getting a new update which includes the latest security patch and other device related improvements.
This update brings April 2019 security patch that provides fixes to 15 new Samsung Vulnerabilities and Exposures items found in the recent software version. The update also installs bug fixes, device stability, and performance improvements.
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The build version of this update is G960FXXS3CSD1 and G965FXXS3CSD1 for Galaxy S9 and S9+ respectively. The update is currently rolling in European countries and will expand to more regions as it proceeds.
If you want to check the update manually please go to the Settings > Software update > Download and install.