Step 1: Download and extract the stock firmware on your computer.
Step 2: After extracting you will find the Firmware, Driver and instruction manual.
Step 3: Install the USB Drivers (skip if drivers are already installed).
Step 4: Follow the instruction manual to flash the firmware on your smartphone.
Take a Backup of your personal data before flashing or installing the firmware on your smartphone. As this helps you to recover the personal data whenever any thing wrong happens.