Did you try to restart your phone?
The only way to roll back is a factory reset.
When you turn your phone off you can press the following button combination:
Volume up + bixby + power off/on and hold it until you get the android recovery menu. You can navigate with the volume buttons and select with the options with the on/off button. Please note that the factory reset wipes all local data (A factory reset is the only way to roll back).
Please note that you require the account that you used to activate your phone to remove the re-activation lock (Its an anti-thief measure). If you don't have the account please tell me, I know one additional way to factory reset the phone that removes this lock if you didn't reset it yet 
It would require navigating though the menu so it might be bugged, which is why I recommend this method.
Fixing? Perhaps, going back to 8.0? No. That requires a factory reset.
For fixing is to start your phone in save mode and see if the issue occurs there (Hold the off button for a few seconds, wait until the menu with the options "Power off, Restart, Emercency" appears. Hold Power Off a few seconds until you get the save mode option.
I really hope that this gets the menu working again (Even if only in save mode ). But its also a good way to give the system a nice kick.
My personal experience was that the phone was slower for a short time after the update - But after a few days everything went smooth for me again (Some optimalisation is going on in the background).
If that option doesn't resolve the menu issue - I would still recommend a factory reset.
