If you’re a Galaxy S8/S8 Plus owner, you can register through the Samsung Members app. In app go to Home > Notices > Register for Galaxy Beta Program. After that, you can download the update by heading to Settings > Software update > Download updates manually.
The update weighs in at around 1,395MB. Although Samsung hasn’t yet officially announced that this is the second phase for the Galaxy S8/S8 Plus Oreo Beta program.