Find the downloaded rollback package in your "Downloads" folder. If it ends in rename it to Move the package to the root directory of your internal storage (outside of any folders). Install the Rollback: Local Update app you installed. in the top-right corner.
Since your phone will be factory reset, you must secure your files first. Downgrade Oneplus 9 To Android 11
Download the specific rollback package for your region (NA, IN, or EU) . Steps to Downgrade via Local Update Find the downloaded rollback package in your "Downloads"