When a beta is available, it will show in the Members app of the applicable Samsung device, which in this case coming up will be the S25 series first. Open the Members app, then with the loaded page, scroll all of the way down to the bottom and select "Beta Program". When that opens, it will either show one is ready to join or it will say "No beta programs are available now.".
IIRC, Samsung made the beta program a default part of the Members app during or after the UI 8 beta program.
In addition to what others have said, when a beta program is available, please be aware that simply having an S25 series device is not necessarily sufficient to be eligible for beta testing.
Your device generally needs to be factory unlocked (not carrier unlocked) and have a model number ending in U1.
1. As with previous One UI Betas, users need to have a Galaxy device in the following markets: South Korea, US, UK, Germany, Poland, India, or Mainland China. 2. Carrier-locked devices are eligible for up to one year of beta updates after the device's release. For example, if their carrier-locked Galaxy S25 phone (which debuted with One UI 7) is enrolled in the beta, they will only get beta updates up to One UI 8.x. Not all carriers are supported, though. 3. Unlocked and Wi-Fi only devices are eligible for up to three years of beta updates after the device's release. For example, the Galaxy Watch 6 is eligible for beta updates up to One UI 8.x (or 9.x since there was no One UI 7 for watches).
