How to Update to iOS 15:
Step 1.) Use the download link below to get access from Apple to update your iPhone to iOS 15 Beta.
Step 2.) Tap Allow from the popup.
Step 3.) Open Settings app - Profile Downloaded.
Step 3.) Then follow the prompts to install configuration beta in Settings app and restart your device.
Last Step.) After your device has restarted, you can receive beta update in Settings - General - Software Update.