The Software version is G960U1UEU3ZRK9 and G965U1UEU3ZRK9 for the Galaxy S9 and S9+ respectively. It has a big size, it is 1790.17MB.
Once signed up for the Beta program, you can check for the update manually by heading into the Settings > Software updates > Download updates manually. This will allow the software to update and load the beta version.
The stable Android Pie update for the Galaxy S9 and Galaxy S9+ is expected to follow in January 2019.