Steps to fall-back from Windows 10 Technical Preview to Windows Phone 8.1:
1. Download the Windows Recovery Tool .
2. Install the Windows Recovery Tool and Launch it.
3. Connect your WinPhone to your PC. If your WinPhone is not automatically detected, disconnect it from your PC and tab on My phone was not detected at the bottom.
4. Once your WinPhone is detected, tap on your WinPhone tile as shown below.
6. Tap continue on the Disclaimer screen.
7. Leave rest to the Tool.
8. Operation successfully completed is displayed on the screen, after the tool is done with the re-installation.
NOTE : Make sure you backup your data.