Unless you’ve purchased Windows 10, you can’t install it on your computer (because clean install requires a product key to insert during the installation), this applies to Windows 7 and Windows 8.1 users, but you can perform a clean install after the upgrade, here is how.
Performing a Clean install of Windows 10 after the upgrade
After upgrading to Windows 10,
1. Depending on the Windows version you use, download 32-bit or 64-bit version of Media Creation Tool.
2. Choose the second option, ‘Create installation media on another PC’
3. Select Language, Edition and Architecture. If you’re using ‘Windows 10 Pro’ or ‘Windows 10 Home’, select the same option from the drop down for Edition.
4. Here you can choose Windows 10 installation media as a USB drive (requires a minimum of 3GB space, go for it!) or you can also download ISO and then burn it to DVD or USB Drive.
5. Once USB drive or DVD is ready, restart your computer
6. Enter the BIOS and set the first boot option as Flash drive or DVD and wait for the installation to begin.
Don’t enter any product key during the clean installation of Windows 10 if you’ve used the upgrade offer, skip it, your PC will be activated automatically online after that.
During the installation, if you were asked for a product key, skip it, since the same edition of Windows 10 was successfully activated on your PC by using the upgrade offer, your PC will activate automatically.
Don’t even think of entering your Windows 7 or 8.1 key during or after the installation, it doesn’t work and not needed.
If you’ve any activation issues, contact Microsoft by opening the Support app in Windows 10.