Allow Retake of a Quiz
On new or existing quizzes, you can allow students more quiz attempts (up to 10, or an unlimited number) and calculate a grade based on their highest attempt, lowest attempt, first attempt, last attempt, or the average of all attempts. You can also put restrictions on whether students are allowed to take further attempts based on the grades made on the previous attempts.
Go to the quiz you wish to edit by clicking Course Activities on the Navbar followed by Quizzes.
Make a new quiz or locate the quiz you wish to have students retake. On the drop down menu of the selected quiz select Edit
Click on the Assessment tab, 
Scroll down to the Attempts area. Set the Attempts Allowed number to the number of attempts you want students to be able to make and select your Overall Grade Calculation. In this example "Highest Attempt" has been selected..
Retake Incorrect Questions Only
When this setting is selected, learners who attempt a quiz more than once can only answer questions that were incorrect on the previous attempt. If you are using Automatic grading, Written Response questions will be marked as 0 and included in future attempts until they are graded manually.
Click Save and Close when you have finished configuring your attempt settings.
For more information on setting up quizzes see: