Unfortunately, a downgrade requires a clean install, so be sure save anything you do not want to lose first.
If you are going back to Windows 7 Ultimate, then you would need to use a Windows 7 Ultimate ISO from the link in step 1 along with your Windows 7 Ultimate product key number.
Since this is an upgrade key for Windows 7 Ultimate, then you would want to use the tutorial below instead when you get to step 8.
Clean Install with a Upgrade Windows 7 Version