The Samsung Galaxy S9 and S9 Plus have been reported not to be in line for official upgrades to Pie until January. In the meantime, some users do have access to a version of the OS, in exchange for some beta-testing. In Korea, the latest of these betas replaces Samsung Experience with One UI.
https://www.notebookcheck.net/New-Korean-beta-ROM-for-Samsung-Galaxy-S9-series-contains-One-UI.362051.0.html