Upon launching Windows Anytime Upgrade, you will notice that the first screen presents two main options: Go online to choose the edition of Windows 7 that is best for you, and Enter an upgrade key. Click the option Enter an upgrade key. From here, you will notice that Windows asks you for a key, similar to the key that you received when you purchased your current version of Windows. If Windows came with your computer, this upgrade key would be similar to the serial key that came with your computer. This key can be found on a sticker, located on the side of your computer's case.
Upon entering a valid Windows Anytime Upgrade key into the Enter an upgrade key screen, Windows will allow you to upgrade to your new operating system. For example, if you are currently using Windows 7 Home Premium, you can enter an upgrade key to Professional or Ultimate. If you are currently using Windows 7 Professional, you can enter an upgrade key to Ultimate.