If you're a teacher or a student who registered with an email address on GrammarFlip, simply click the "Forgot Password?" link (on the following page: https://app.grammarflip.com/), enter your email address, and click "Retrieve Password." In doing this, the window will slightly change to reveal a field in which you can enter a pass code token. Check your email, and enter that pass code token that is provided to you to access your account.
If you are a student who has registered without an email address, contact your teacher to reset your password.