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 Windows 7 installation DVD for your version, or unlock all versions in any Windows 7 installer to reinstall your licensed version with the key on COA sticker. Follow these steps to clean reinstall Windows 7: Reinstalling Windows 7