Follow these steps:

  1. Go here to edit your Instagram profile (login with Facebook if you have to):
  2. Enter an email address & a username if those fields are empty
  3. Log out of Instagram
  4. Go here to reset your password and enter your email:
  5. Open your email and press the link in the email you should have received from Instagram
  6. Set a new password
  7. Go to Preview and use your username & password to login