How can I install iOS 15 and above as a beta profile on iPhone 6?

beta profile ios 15iOS 15.8.x is only available for iPhone 7, iPhone 7 Plus, iPhone SE (1st generation), iPhone 6s6s Plus and iPod touch (7th generation).How to download and install iOS 15 public beta (now on 2) software for iPhone 6S through iPhone 12 Pro Max (includes 7, 8, X, XS, XR