Reset and change your passcode by heading to the Account tab and tapping the gear icon in the top right corner. Select 'Passcode & Touch/Face ID', then flip on the passcode toggle. You'll be prompted to enter a new passcode.
If you are unable to log in to your account and need to reset your passcode, you will need to enter the wrong passcode ten times in your app. You will then be prompted to log in with your email and password.