Go back to the page where you registered, deregister then connect your phone to PC/Mac via Smart Switch. After plugging in your phone using SW, an update for the previous version(Android 11, One UI 3.1) will then download allowing you to rollback.
Opting out of the program would require you to back up device and use smart switch to restore Android 11 software first and then data back. May we suggest you backup all your data first, Master reset device on beta software and restore data to see if device performs any better as you lose all data on device and need to restore them in both cases.
