You should be able to uninstall 2.5 and reinstall 2.0.1. From your message you have the original installer and for future reference, you can download it from the Formativ 2.0.1 download page, http://www.advansyscorp.com/formativ_download_20.htm.
Which operating system are you using? When you uninstall, perhaps check that all files are removed from the Formativ program folder, C:Program FilesAdvansysFormativ or C:Program Files (x86)AdvansysFormativ. The registration utility application is frmrmg10.exe.