Upgrading to windows 10 from 8.1 after factory reset

Hi Avijit_08, you can reinstall Windows 10 at any time and it will not cost you anything !

Since you previously had Windows 10 installed and activated on that PC during the installation process skip the steps which ask for a product key and select the option 'I am Reinstalling Windows 10 on this PC', and activation will not be an issue, your PC will have a digital entitlement stored on the Microsoft Activation Servers

Make sure you install the same Edition of Windows 10 (Home, Pro . . . etc.) you previously had installed to avoid Activation issues

Click this link:
https://www.microsoft.com/en-us/software-downlo...
to download the Media Creation Tool (Click on Download Tool Now), with that you can download the latest Windows 10 ISO (Select Create Installation Media for Another PC), you can create a bootable USB flash drive (min 8GB) using that tool

Then, Boot your PC from the Installation Media you just created (change Boot Order in your BIOS) to begin installing Windows 10

If you have problems booting from a boot disc, you may have UEFI BIOS:
Insert the Bootable Installation Media, then go into your BIOS and make the following changes:
1. Disable Secure Boot
2. Enable Legacy Boot
3. If Available enable CSM
4. If Required enable USB Boot
5. Move the device with the bootable disc to the top of the boot order
6. Save BIOS changes, restart your System and it should boot from the Installation Media

You can carry out a Windows 10 reset instead of a factory reset. Go to Settings > Update and security and select Recovery. Select Restart now and follow the instructions.

If you want to go back to Windows 8.1 (which isn't necessary) and then install Windows 10 again you can download the Media Creation Tool and make your own Windows 10 boot media.

http://www.microsoft.com/en-us/software-download/windows10

After you have created either USB media you can upgrade again just by simply running the setup.exe from the USB drive while running Windows 8.1.

Upgrading isn't the best way to install Windows 10, it's much better to clean install Windows 10 by booting from the USB media. Windows 10 will preform better. There is no need to worry about a licence key the device has a digital licence and will activate once connected to the internet.

Do a direct clean install of Windows 10 rather than reinstalling windows 8.1.make sure to install the same version as was before.
https://answers.microsoft.com/en-us/windows/for...