Not happy with the Windows 10 Technical Preview? Here is how you can drop back to Windows Phone 8.1
To bring Windows Phone 8.1 back to your handset, you will need your phone, a USB cable, a Windows PC and the Windows Recovery Tool which is downloaded to your PC. Use the USB cable to connect your phone with the computer. The phone should be automatically recognized by the computer. If not, disconnect the phone from the PC and click on 'My phone was not detected' at the bottom of the screen; follow the instructions on the display.
Once your phone model is detected by the computer, click on the name of the model on your PC screen. The next screen will show software versions; click on the reinstall button. You will then see a disclaimer suggesting that you back up the phone. Click on continue. The next step has the recovery tool downloading and installing the firmware. It is important that you do not disconnect the phone from the computer while this is happening. When completed, you will see a notification that says "Operation successfully completed." Oh happy day!
Don't consider it a failure if you need to go back to Windows Phone 8.1 from Windows 10 Technical Preview. After all, we are not talking about the final version of the new Windows build here. Once Microsoft kills all bugs dead and releases the finished Windows 10 for Mobile, you should try again to update your Windows Phone device to the latest and greatest build of Microsoft's mobile OS.