Microsoft no longer sells "Anytime Upgrade" keys through their store. If you have a valid retail product key for a higher edition of Windows 7, you can use it for the upgrade.
Introduced as a simplified way to jump between editions (e.g., from Home Premium to Ultimate), Windows Anytime Upgrade is built directly into the operating system. Unlike traditional upgrades that require a bootable USB or DVD, WAU works by unlocking features already present on your hard drive using a specialized product key. Windows Anytime Upgrade - Microsoft Wiki | Fandom windows anytime upgrade key for windows 7