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Amarnath Subramani
If you advise that, it's you don't know the functioning of Windows Device Recovery Tool (WDRT). OP want reset his device, but he prefers do it without rollback to Windows Phone 8.1. In that case, hardware reset is the only solution. For Lumia devices, WDRT download and install the latest package version avalaible from the Microsoft Azure repository servers, the package being determined by the CTR writed from the Device Provisioning Partition (DPP). And for non-native Windows 10 Mobile devices (like Lumia 640), the latest package versions contains Windows Phone 8.1.