Using the Software Recovery Tool will erase all content stored on your phone.
Download and install Windows Phone Recovery Tool from http://go.microsoft.com/fwlink/?LinkID=525569
Start the application.
Connect your phone with USB cable and the application will be connected to the phone.
Check the phone info details and click reinstall software. If the phone is not connected within a minute you can try to restart your phone while USB connected, press and hold the Power and Volume down keys simultaneously and release the pressed keys when phone vibrates.
Click on My phone was not detected.
Choose your phone manufacturer.
Click on Install software.
Software package installation successful completed. Click Finish to exit.