The Galaxy S9 is just starting to make its way out to pre-order customers, but Samsung is wasting no time getting the device updated. The first OTA for Samsung's latest phone is hitting devices, but deployment will probably vary by region and carriers. When you do get the update, it'll bump your phone to the latest security patches that just rolled out to Pixels the other day.
According to reports, the OTA has been spotted in Germany and clocks in at 242MB. The only obvious change is an update to the Mach 2018 security patch level from February 2018 on the shipping software. At over 200MB, there might be some other small tweaks in there. It's still based on Android 8.0, of course.
The version number for the update is G960FXXU1ARC5 and G965FXXU1ARC5 for the GS9 and GS9+, respectively. You can check for updates in the settings menu as soon as your phone arrives. No promises, though.