Each quiz is part of the lesson that it is based on and can be taken as many times as you would like. The quiz(es) for each lesson can be found at the bottom of that lesson page as shown below.

Quizzes can also be accessed from the expanded course content on the main course page or from the Left hand side menu on the lesson pages.

Within the quiz you can rewatch each video until you are sure of your answer, and you can take each quiz as many times as you like, even after you have passed it.