The app does not support the ability to reset your progress, unless you delete your account and then sign-up again. If you wish to do this, you can delete your account from within the app via the More menu, just scroll to the bottom and select the Delete my account link. Once you have deleted your account, you will be able to sign-up for a new account using the same email address (or an alternative one, if you prefer). As this will be a brand new account, your data will have been cleared and therefore your progress will have reset.