https://wccftech.com/download-ios-15-and-ipados-15-rc-right-now-without-developer-account/

No need to pay Apple anything if you want to download iOS 15 and iPadOS 15 RC for iPhone and iPad. Here's how you do it.

Apple's iOS 15 and iPadOS 15 RC updates have been seeded to developers and public beta testers. If you have a previous beta installed on your iPhone and iPad right now, then the new update can be grabbed over the air by going to Settings > General > Software Update.