Microsoft today revealed the upgrade path for Windows 10. If you are on Windows 8.1 S14 or Windows 7 SP1, you will be able to upgrade via Windows Update. All other Windows 7 and Windows 8 customers need to download Windows 10 ISO for upgrade. Microsoft also confirmed that Windows RT users won’t be able to upgrade to Windows 10. As previously announced, Microsoft has promised a update for Windows RT users which will bring a subset of Windows 10 features to their devices.
As you can see, Windows Phone 8.1 users can also update your device to Windows 10. If you are on Windows XP or Windows Vista, you can still install Windows 10 with ISO media.