MAC Mini High Sierra Upgrade Issues

I recently upgraded my MAC mini to High Sierra from my previous version of OSX-MAC 10.9.5. This was recommended to perhaps solve the problem of my webcam not functioning properly with my MAC mini (Yes the webcam was recommended for my device).

Now, I'm having all sorts of compatibility issues with my other devices like my TASCAM US-1800 Audio Interface (not compatible with this operating system). And now my display keeps changing and expanding off the screen, and I have to constantly change the options to actually see my Status bar at the top.

This is so frustrating. I have been trying to figure out a way to return to my previous operating system, but to no avail. Forget the webcam! I just want my previous version back so that my other devices and software will work like it used to.

HELP!