The Software version is G960FXXU2ZRL5 and G965FXXU2ZRL5 for the Galaxy S9 and Galaxy S9+ respectively. It has a big size, it is 1765.45 MB. It comes with the December 2018 security patch.
How to participate in the One UI Beta Program?
For participating in the beta program, you need to login/register to the Samsung Members application present on your device.
After login, go to notice section and click the notice image “Registration of One UI Beta Program”, and then submit the “One UI Beta Program” application form. If you have a Samsung Account and you meet all participation criteria, the entire registration process will be completed.
[Note: if you do not have a Samsung Account, you can create a new Samsung Account from – https://account.samsung.com/membership/signUp.do.]
Once signed up for the Beta program, you can check for the update manually by heading into the Settings > Software updates. This will allow the software to update and load the beta version.