Next and Back buttons can be added in a quiz to provide a flexible flow within questions.
In any questions added, click on the Edit on Builder button.
Click on the Popup Properties menu located at the left column.
Turn on the Use Next Button and Use Back Button toggles to enable the buttons.
Note: the Back button is enabled throughout the quiz, however, the Next button will be enabled when a question is not a required question. If the question is required, the next button is displayed but disabled (grayed out), and can be clicked once a question is answered.