When Do You Need This?
The Install Tool is directly accessible at <yourdomain>/typo3/install.php – all you need for this is the Install Tool password, no username required. Alternatively, it can also be opened from within the regular TYPO3 backend, provided the logged-in user has both admin and maintainer rights.
If the password is no longer known or the backend is unreachable for other reasons, the only remaining option is via the web server: to reset it, you'll need FTP or SSH access to your hosting.