Installing Windows Mobile 10 (wm10) on your Windows Phone

This has worked for me on my Nokia Lumia 720.

  1. Back up all the data and pictures on the phone
  2. Hard reset the phone (For a beta OS its better to start from a clean phone)
  3. After hard reset, DO NOT restore from backup. Set up as new phone.
  4. Download Windows Insider App
  5. Select fast ring and complete all steps until the phone reboots
  6. Check for updates in Settings>Phone Updates
  7. Download and install update for Windows Phone 8.1 Update 2
  8. Again check for updates in Settings>Phone Updates
  9. Download and install Windows phone 10 Technical Preview
  10. Very important : Hard reset the phone
  11. Now install all the apps that you need, from the store.

http://forums.windowscentral.com/windows-10-mobile-insider-preview/351062-guide-how-install-windows-10-mobile-insider-preview.html