If you have a laptop, download samsung smart switch on it,open and give all the permission needed and then connect the phone using the USB-C charging cable preferred samsung charging cable to the laptop, if you are using the s26ultra model u will get the option to downgrade to the last stable update. Before you downgrade it backup your data in the laptop itself in the smart switch. Also make sure to follow the instructions properly and use the samsung cable for better and faster transferof data and need good network connection for this to work. It took 6 tries for me to downgrade it back to stable 8.5 UI. You will lose all the features from the beta 9 update and will need to reconfigure your settings.
