Hi Lalit. I'm Greg, an installation specialist and 9 year Windows MVP, here to help you.
I believe you said you wanted to be able to download Windows to reinstall it on PC's which came with it or had the shipped version upgraded. Is this Correct?
You can always reinstall Windows as often as you want and in any way you please.
I would follow the illustrated Clean Install steps in this tutorial which compiles the best possible install that will stay that way as long as you stick with the tools and methods given: http://answers.microsoft.com/en-us/windows/wiki...
To create Windows 10 Installation Media (if necessary on another PC) install the Media Creation Tool and follow the directions here: http://windows.microsoft.com/en-us/windows-10/m.... Uncheck the box for Recommended Settings to choose the exact version and bit rate for the target PC only.
Insert media, boot it by powering up PC while pressing the BIOS Boot Menu Key: https://www.sysnative.com/forums/hardware-tutor...
If the media won't boot you may need to enter BIOS/UEFI Setup (pressing key given in chart in link above) to turn off Fast Boot or Fast Startup first.
Choose the boot device as a UEFI device if offered, on second screen choose Install Now, then Custom Install, then at the drive selection screen delete all partitions down to Unallocated Space to get it cleanest, click Next to let it create needed partitions and start install - this makes it foolproof.
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You will get and keep the best possible install to the exact extent you stick with the steps, tools and methods in the linked tutorial. It's a better install than any amount of money could buy and a great learning experience that will make you the master of your PC because you will learn everything that works best and how to apply it with your own hands.
I hope this helps. Feel free to ask back any questions and let us know how it goes. I will keep working with you until it's resolved.