accidental windows upgrade

My nephew accidentally upgraded my windows 7 home premium to windows ultimate. A pirated one of course. I would like to revert the system to my legal version. Please let me know how could do this without re installing the OS.

Welcome to Seven Forums.

I'm sorry but you'll have to do a clean install. Even a legal copy of Ultimate only has downgrade rights to downgrade to Vista or XP.

Accidentally? I wish I had such powers.
If I did, I'd "accidentally" become rich and famous.

There is a workaround to change to a lesser version using an in-place Upgrade or Repair Install. However you need to find out how the bootleg was activated because if it has a BIOS modification cracktivation then it will need to have the HD wiped to be rid of it with certainty. SSD - HDD Optimize for Windows Reinstallation

If this is a factory preinstall then you can possibly run the Recovery partition to recover to Factory condition, or boot the Recovery disks which you made when setting up the computer or order from Tech Support.

Absent these choices you'll need to find a Win7 installation DVD for your version, or unlock all versions in any Win7 installer to reinstall your licensed version with the key on COA sticker. Follow these steps to clean reinstall Win7: Reinstalling Windows 7

Has he upgraded with a pirated windows 7 ultimate DVD or used the Windows Anytime Upgrade with a pirated key?