Follow these steps:
- Go here to edit your Instagram profile (login with Facebook if you have to): https://www.instagram.com/accounts/edit/
- Enter an email address & a username if those fields are empty
- Log out of Instagram
- Go here to reset your password and enter your email: https://www.instagram.com/accounts/password/reset/
- Open your email and press the link in the email you should have received from Instagram
- Set a new password
- Go to Preview and use your username & password to login
