Apple is now on its second round of betas for iOS 15.1, tvOS 15.1, watchOS 8.1, and iPadOS 15.1, with developers now able to download the new builds.
The latest builds can be downloaded by developers in the beta-testing scheme via the Apple Developer Center, or as an over-the-air update for hardware already using earlier beta builds. A public beta version of the releases is expected to start arriving soon, and will be available through the Apple Beta Software Program website.
The second betas for iOS 15.1 and iPadOS 15.1 have build number 19B5052f, while tvOS 15.1 is on number 19J5552f. WatchOS has build number 19R5551d.
Both AppleInsider and Apple itself strongly recommend users don't install the betas on to "mission-critical" or primary devices, as there is the remote possibility of data loss or other issues. Instead, testers should install betas onto secondary or non-essential devices, and to make sure there are sufficient backups of important data before updating.