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Kicking off the consumer-focused Windows 10 event in Redmond on Tuesday, January 21, Microsoft operating system chief Terry Myerson announced that current users of Windows 7, Windows 8.1, and Windows Phone 8.1 will be able to upgrade to the new operating system for free as long as they do so within one year of Windows 10's launch.
Once you’ve claimed the upgrade, it's permanent, and Microsoft will keep you updated for the supported lifetime of the device.