When the phone stops functioning properly in certain cases (such as fails to restart or switch off, update the system, wipe data ), you can fix the issues under Recovery Mode. After your phone turns off, press and hold both the Power and Volume Down buttons, until the phone enters Recovery Mode. You can download the firmware to your phone storage, then click Install from storage device > From phone storage, find the firmware you’ve downloaded and click it to start updating.
* Back up your data before updating the system.
* Plug your phone into power or keep the battery level over 40%.
* System update takes 3-5 mins. Do not force power off your phone during the update.