To reserve your free upgrade, from your existing Windows 7 or Windows 8 PC click on the Windows 10 icon located in the lower right side of the system tray. If you don't see the Windows 10 icon try running Windows Update and install any available required and recommended updates for your existing Windows 7 or Windows 8 PC.
Some great articles to read about Windows 10:
Windows 10: Nearing the Finish Line (Part 1)
Windows 10: Nearing the Finish Line (Part 2)
Windows 10: Nearing the Finish Line (Part 3)
Read more about Windows 10 new features here - http://www.microsoft.com/en-gb/windows/features