Since the downgrade , use the OnePlus Switch app to back up your contacts, messages, call logs, and apps to an external USB drive or PC. Manually back up photos and documents to Google Drive or a computer.
Modern OxygenOS builds hide the manual local installation menu unless Developer Options are active. Open on your OnePlus 9. Scroll down and tap About Device . Tap on Version to reveal the Build Number. Tap Build Number seven times consecutively. Enter your lock screen PIN or password when prompted.