2 Answers
If you own the router, go to their support page and lookup the default login/pass. If that doesn't work, you may need to do a hard reset. If you do a hard reset, you'll lose all current special settings if any are applied.
If you don't own the router and are renting it from your internet service provider, you can contact them with necessary changes. Perhaps they'll give you an admin login/password.
Know the answer? Login or sign up for an account to answer this question.