If you’ve purchased a Windows 7 PC anytime since June 2nd and through January 31st, 2013, you can register for the Windows Upgrade Offer and get a promo code to purchase and download Windows 8 Pro for $14.99 USD. You will receive a promo code via email. You then go to Windows.com to buy a Windows 8 Pro upgrade.
A small application named Windows 8 Upgrade Assistant will assist you from here. When you get to the point in the process where you are asked to confirm your order, you are given the opportunity to enter the promo code you were sent by registering for the Windows Upgrade Offer and the price of Windows 8 Pro will then drop to $14.99 (U.S.). Then you proceed with the rest of the process and upgrade your PC!
Source: Windows blog
Note: You can burn a DVD and then perform a clean installation of Windows 8 with the upgrade licence. So save yourself some money and buy the upgrade licence instead of the costly retail box of Windows 8.