If you registered using your email address (i.e., not using the Facebook option), click on Forgot password? from the login screen.
On the next screen, simply type in your registered email address and click Reset Password. You will then receive an email. Click the password reset link and enter in your new password. Confirm the password and you should be good to go again.
If you are having issues logging in to Facebook using the application, follow this guide to reset your password: Unable to Log In Using Facebook App. Don’t worry; you won’t lose any settings.