The latest beta is for Apple's iOS 15.2, and it's available now for testers and developers.
Then you should be able to find the latest beta version by going to Settings > General > Software update and selecting Download and Install. Once you're participating in the program, you'll need to download the configuration profile from Apple. Note that beta versions of software are buggy and not optimized, so we only recommend downloading betas on an extra device, not your primary phone, tablet or laptop.
In order to test the beta versions of Apple software, you first need to sign up for Apple's Beta Software Program.