You need Smart Switch software on a Computer.
I connected my S10 with Android 10 beta on it, and the smart switch said it needs to do some updates. Once it did those, it had a message under the device name saying I can go back to the previous version.
I did that, and it loaded Android 9 for me.
As soon as your phone boots up on Android 9, switch off automatic updates and pause the update if it already started downloading, because it immediately starts downloading and I cant find a way to delete what it's already downloaded.
You also get an annoying persistent notification about an update that I had to silence/hide.