Hi Dave,
I’m sorry you are having problems with the latest update. Our developers are alredy aware of the issues and working hard on fixing them ASAP.
But for the time being, if you need to get back to 7.5.20 you can actually uninstall just the Cubase 7.5.30 update in your Windows Control Panel / Programs and Features / Uninstall a Program. On the left hand side of that window you will find an option called “View installed updates”. Click there and look for the Steinberg Cubase 7.5.30 Update and just uninstall it. That should bring you back to 7.5.20
All the best