How to reinstall Windows 10 after Factory Reset to 8.1

Create a system restore point before doing anything else. Select the keyboard shortcut Win+S and type Create a restore point. Click the Best Match at the top of the dropdown menu and click the Create button.

Are you using an ethernet cable or wireless to connect your computer to the router? An ethernet cable connection is faster and more reliable for large downloads. An ethernet cable is an inexpensive purchase.

Connect to a mains electricity supply if your computer is a laptop.

Disable your anti-virus and firewall whilst upgrading.

I suggest you download the Windows 10 Media creation tool ISO and do an In-place upgrade.

Download the updated Windows 10 Media creation Tool:
http://www.microsoft.com/en-us/software-download/windows10

How to download an ISO using Media creation Tool:
http://www.tenforums.com/tutorials/9230-windows-10-iso-download.html

How to perform an In Place upgrade using Method 1:
http://www.tenforums.com/tutorials/16397-repair-install-windows-10-place-upgrade.html#step6

When you upgraded from a previous version of Windows or receive a new computer preinstalled with Windows 10, what happened is the hardware (your PC) will get a digital entitlement, where a unique signature of the computer will be stored on Microsoft Activation Servers. The Windows 7 or Windows 8 genuine license you were previously running will be exchanged for a diagnostics key.

Anytime you need to reinstall Windows 10 on that machine, just proceed to reinstall Windows 10. It will automatically reactivate.

So, there is no need to know or get a product key, if you need to reinstall Windows 10, you can use your Windows 7 or Windows 8 product key or use the reset function in Windows 10.

If you are prompted to enter a product key, click 'I don't have a key' and 'Do this later'

Download the Media Creation Tool Now

Right click the MediaCreationTool.exe file then click Run as administrator.

Note: How to verify if you downloaded or installed the Windows 10 Anniversary Update

Select the option Upgrade this PC now if you want to upgrade right away, of if you want to download a ISO image or create a bootable copy on a USB thumb drive, choose Create installation media for another PC then click Next

A new option has been added to the setup screen for users reinstalling Windows 10 on a previously installed and activated system. If your system has a Digital License attached to it, Windows 10 will automatically upgraded: