Update (Feb. 2016) – Nokia Software Updater (NSU) is not available now as Microsoft first transformed it into Lumia Software Recovery Tool (LSRT) and then into Windows Device Recovery Tool (WDRT). So please refer to this updated guide which explains the use of WDRT to update/reset WP/Lumia 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