Teachers can allow students to retake an exam after resetting the exam. Resetting an exam for a student will delete all answers previously submitted by the student.
Click the module access icon > Academics > Examination > Online Exam > Reset Exam to open the Reset Exam page.
Select the batch and the exam.
Uncheck the students who you do not want to retake the exam.
Click the Reset button.
For more information, view this: