Six years ago, I bought dell inspiron n5010 laptop with inbuilt
genuine Windows 7. Now I want to download genuine Windows 10. Here in
according to Windows 10 update assistant it is showing that "This PC
is compatible"(as highlighted in red box).
You should use the update assistant to download and install Windows 10. You can if you want, choose not to keep your personal files, and the Windows 10 installation will be identical to a "clean installation" that comes from formatting your HDD within the installation environment. You should upgrade to Windows 10 from within your current Windows 7 installation. If you don't choose to do that, you will need the key printed on the COA sticker, attached to your Windows 7 device in order to install and activate Windows 10.
So if I download Windows 10 ISO from any 3rd party website, will my
system activates it's license?(Because product key lies somewhere in
Windows 7 license keys are not stored in the ACPI table. Windows 7 devices, devices that came with Windows 7, are physically incapable of storing the license key within the ACPI table. ALL OEM Windows 7 devices were required to have the license key printed on the COA sticker attached to your device, by Microsoft in order for the OEM to indicate the device came with Windows 7.
For the record, I am not able to retrieve product key because it is
SLP encrypted(System Lock Precision according to Microsoft guy which
can't be retrieved by any means and if gets retrieved somehow, then it
won't work.) So don't suggest me to retrieve by any method.
You were told bad information. The license key that you would use, if it was even required to be used, would be on the COA sticker attached to your device.
If you are currently eligible for the only free upgrade offer that does exist today (due to a disability) , you don't need your Windows 7 license key in order to activate Windows 10, simply upgrade to Windows 10 and it will automatically activated. The free upgrade everyone was eligible for expired in July 2016.
If I download Windows 10 ISO from third party website (like
www.getintopc.com), flash into USB and install it, then will it be
going to ask me for product key?
If you upgrade to Windows 10 from Windows 7 you don't need to know your Windows 7 product key. ShowKeyPlus is the only program, I know that will display your Windows 7, for your current Windows 7 installation. You should not, under any situation download Windows 10 from any other source other than directly from Microsoft, you can do that here.