Students can submit an assignment using the Submit button on their assignment. Afterward, they cannot make changes to their response. As an instructor, you can unsubmit an assignment.
Method 1: Unsubmitting for a Single Student
From the Scores page of the assignment, scroll to the student's name and click the three dots to the right of Grade.
Click Unsubmit Assignment. This will change the status back to In Progress and allow the student to keep working.
Method 2: Unsubmitting for Multiple Students
From the Scores page of the assignment, scroll to the student's name and place a check in the box of all of the students that you wish to unsubmit.
If you're trying to unsubmit for everyone, check the box at the top of the table to select the whole class.
In the Bulk Actions menu, select Unsubmit Assignments, then click Apply.