A Product Key is a unique 25-character alpha-numeric code used to redeem and activate your Office product. You cannot use a product key for an older Office product (or a different product) to redeem your new copy of Office.
To find your Product Key, look for the 25-character alpha-numeric code on the back of the card inside your Office product packaging. Sometimes the Product Key may be printed on your retail receipt. If you purchased Office from a PC manufacturer, your Product Key will be located somewhere in your PC packaging. Sample Product Key format: XXXXX-XXXXX-XXXXX-XXXXX-XXXXX
Get information on locating your product key.