ko888 :
Google "How to Uninstall Windows 10 Technical Preview".
All of those merely tell you how to uninstall any updates that have been applied to the tech preview.
NOT how to go back to whatever it was before.
I went through ALL the steps, and it merely undid whatever updates had been applied to the Win10 Tech Preview.
Oh, the joys of doing this on a PC that does not matter....That's what a test box and early beta OS are for.
It does NOT reinstall the previous Win 8.1.
@Kevin M - depending on how you installed it and what exactly was there before, your Windows 7 key may well be long overwritten. It does not exist any more on that drive.