After a student completes and submits an assignment, you can choose to accept or reject the assignment. Students cannot edit their submitted work after you have accepted the assignment.
Click the module access icon > Academics > Assignment to open the All assignments page.
Select the subject for which an assignment was given.
Click the title of the assignment.
Click on Answered to view list of students who have completed and submitted the work.
Click the name of a student whose assignment you want to evaluate
After going through the student’s work, click the Accept answer button to accept the assignment, or click the Reject answer button to reject the assignment.
The student will be notified if the teacher has accepted or rejected the assignment. If an assignment is rejected, the student has the option of resubmitting the assignment after making the changes instructed by the teacher.
For more clarifications, please watch this.