Apple has released the first public betas for macOS Big Sur, iOS 14, iPadOS 14, tvOS 14, and watchOS 7. The complete versions should arrive this fall.
The public betas are available on Apple's Beta Software Program website. Just remember that beta software, so proceed with care. Apple does public betas in order to increase the size of the pool of testers. Its recommends to keep back-ups of your files before installing a public beta.