1- Install Qualcomm Driver.
2- Make sure your device getting identified in device manager as Qualcomm HS-USB Diagnostic 9006.
3- Use Power or Volume up or combination of both as boot key.
4- Run msm8x39 Download tool.
5- Click on verify, if it display message ok then you are ready to go.
6- Click on start to Flash.
7- Download Progress will be displayed.
8- When completed, Download complete will be displayed.