1. Click on the assignment that you want to allow multiple attempts for.
2. Click "Edit" in the top right corner.
3. Under "Max graded attempts," enter the maximum number of graded attempts you would like your students to have on the quiz. For example, if you set this to 3, students will be able to take the quiz 3 times, and their highest grade out of the times they've taken the quiz (up to three) will be the one displayed in the gradebook. This works across LMSes as well on Boundless.com.