Steps:
-> On you iPhone / iPad, open SAFARI then type BETA.APPLE.COM
-> TAP "Sign Up" if you don't have APPLE ID, else TAP "Sign In"
-> On next screen, SIGN IN your APPLE ID
-> TAP "Enroll you iOS Device"
-> TAP "Download Profile"
-> TAP "Allow"
-> TAP "Install"
-> TAP "Install Profile"
-> TAP "RESTART"
-> After RESTART, go to SETTINGS then TAP "General", then TAP "Software Update"
-> NOW, you'll will see the LATEST iOS PUBLIC BETA Version
-> You choose to DOWNLOAD & INSTALL the latest iOS Public Beta.