https://wccftech.com/download-ios-15-1-and-ipados-15-1-beta-3-now-available/

Developer can now download and install iOS 15.1 and iPadOS 15.1 on their iPhone and iPads. The update comes a week after beta 2's release.

If you already have the previous update installed on your iPhone and iPad, then you can go ahead and download the latest version over the air. All you need to do is go to Settings > General > Software Update and then tap on Download and Install.