How to repair a bricked phone or downgrade the OS
Downgrade the OS using Nokia Software Recovery Tool
If your Nokia (Lumia) device is not responding to commands (or not turning on) you can try fixing it by yourself. All you need is a program called ‘Nokia Recovery Software Tool’ which needs to be installed on a PC (Windows 7 or above) before connecting the Lumia to the desktop computer. Using this program you can download and install the latest official OS version on your bricked phone and you will most likely get it working again without sending it for repair.
Follow the steps:
1) Download the Recovery Software Tool on your PC from here
2) Open the program on your PC
3) Connect the phone to the PC using USB cable
4) Select the OS version you want to install. All data and files stored on your Nokia phone will be erased. If possible make a backup.
5) The OS will be downloaded in 10-20 min
6) Your phone will start installing the software
Do not disconnect the cable before the installation is complete