First things first: the ability to retake an assignment only exists in Courses. If you've created an assignment within a Test Prep product, students can't retake it—so make sure they know how to complete the assignment correctly on the first try!
If you're looking at an assignment within an Online Course, it’s easy to allow your students a redo.
As a teacher, you can allow your class to retake or resubmit manually graded activities, auto-graded activities, and tests/exams. If you choose to allow retakes, it will open up access to all students in the class. Students who choose to retake an allowed item will have their previously submitted work and any scores associated with it overwritten by the most recently completed version. Please make sure they understand that if their second attempt receives a lower score that the lower score will be their grade.
Go to the "Manage" tab of the relevant classroom.
Scroll down to "Class Settings" and choose which types of activities you'd like students to be able to retake or resubmit. If a box is checked, the student CAN retake/resubmit. If the box is not checked, the student CANNOT retake/resubmit. Please note that whatever you choose applies to all the content in that course.
Click "Save Settings," and you’re good to go!