Update (Feb. 2015) – NSU is not working now as Microsoft has transformed it into Lumia Software Recovery Tool (LSRT). So please refer to this updated guide which explains the use of LSRT to flash/update/reset Nokia phones.
There are many scenarios when a phone needs to be reset/flashed – like a dead/bricked phone, frequent restarts, restart-loop issue, WP8-Lumia cog-wheel issue etc. Quite a few tools are available for general public which can be used to reset/update/flash firmware of a Nokia phone like Nokia Care Suite, Phoenix, JAF, NSS etc. However with the change of Nokia’s firmware download policies and shifting focus to Windows Phone Lumia devices, older methods are getting obsolete. Most Lumia phones (specially newer ones like Lumia 920, 925, 525, 625, 928, 1020, 1320,