Rolling back to Windows 8.1 is far easier than you'd expect, but there is one caveat...
YOU ONLY HAVE 30 DAYS AFTER THE WINDOWS 10 INSTALL TO DO THIS!
All you need to do is go to go into the Settings menu, Update and Security, then Recovery, and you'll see a button like below, that says "Go Back to Windows 8.1"
Thats it Simple!
If you do do this, you ARE able to update back to Windows 10 whenever you want, as your device has been qualified for the free upgrade!