Do OTA, make sure you have full battery and leave it for 40mins as it does take a lot of time.

If ya dont want to do OTA then id go for the flasing software (not NSU) search the forums for it cos im a bit too busy, will try and link it later if noone else can help.

Ive seen a lot of guys have issues with the NSU software but that was a good few months ago and there is a new version out now.