I recently did the software update on my Note9 which upgraded it to Android Pie and I hate it. The horizontal app switching is not intuitive for one handed use and the clock being moved to the left hand side of the screen just hurts my brain. I want to go back to Oreo.
There's no way to go back via Samsung unfortunately. You can, however, install it yourself. If you google it (I'd recommend somewhere like XDA forums) you'll be able to find guides and the files you need to do it.
Depending what you do you may void your warranty though so be careful and do your research first
There are ways, but unfortunately none that you will like
Basically you have to do a factory reset, and then use software on your PC called Odin to put the old ROM (a downloaded version of android) in place of the other one.
It is moderately complicated, but following steps online (just google "downgrade android version") you can do it with no trouble.
Like the previous response said, just search around for the exact steps, as it is very important you use the correct ROM for your device and region or you will have headaches.
Let us know how you go!