Before we get started I just want to let you know that check whether your device is compatible or not. Click here to see further details. So I assume your Lumia is on the list for Windows 10. Here we go:
- Download the Windows Insider App from the Store
- Launch the application and Login with your Microsoft Account, and make sure you have enrolled yourself with the same Microsoft Email you used to Login into the Windows Insider app. You can register yourself here insider.windows.com
- Choose between Insider Slow or Insider Fast, depending on how quickly you want to get updates on your Lumia
- Accept the confirmation
- Tap the check mark
- Go to Settings
- Hit Phone Update menu
- Click Check for Updates
- Now phone will download the new update and will restart couple of time, dont get tensed. Be Patient and relax.