Windows Phone 8.1 Update 2 brings anti-theft Device Lock
Windows Phone 8.1 Update 2 | Windows Central Jun 04, 2015 Update to Windows 8.1 from Windows 8 - Windows Help Jul 13, 2020
Jul 30, 2014 Windows 10 Mobile: Now you can update your phone from your Oct 23, 2017
Windows Phone version history - Wikipedia
May 02, 2014 Microsoft announces Windows Phone 8.1 Update 1 (GDR1) - Neowin Jul 30, 2014 Windows Phone 8.1 Preview Gets Another Update Jun 02, 2014