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 switch the assignment status to Request revision.
Go to My Classes and click on the student's class.
Click the assignment name to open the Scores view for that assignment.
Scroll to the student's name and change the Status to Request Revision.
Once that change is made, the student can continue to work on their responses and submit their work when finished.
Note: The option to Request Revision is not available for an assignment if a student has not opened/viewed the activity. These "In Progress" statuses can only be changed to request revision after the assignment has been closed.