'On Windows you can roll back without uninstalling/installing anything: Control Panel → Programs and Features → View Installed Updates → Click the Cubase 9.5.30 entry and click uninstall.
If needed 9.5.20 can be found here:
download.steinberg.net/downloads_software/Cubase_Pro_9.5_and_Cubase_Artist_9.5/9.5.20/Cubase_9.5.20_Update_win.exe
download.steinberg.net/downloads_software/Cubase_Pro_9.5_and_Cubase_Artist_9.5/9.5.20/Cubase_9.5.20_Update_mac.dmg’