The new Galaxy smartphones have been regularly updated ever since day one of their launch. The devices already got the Samsung Security Patch for March, 2018, while an update two weeks ago targeted some stability issues regarding the facial recognition feature and fixed some problems surrounding the Samsung Pay service. It's not a surprise that Samsung has been lightning-quick regarding Galaxy S9 and S9+ updates, as both phones have been out for less than a month.
To download the new update, go to Settings --> Software Updates. It's a 400ish MB download, so you better be connected to a Wi-Fi network.