Step 1 : Go to 'Assignments', click on the name of the assignment. 

Step 2 : Click on 'Submissions', click on the check box to mark it submitted, if not submitted you can right remarks. You can also exclude student for submission by clicking on remove icon.

Once done click on save.

You can update the defaulters or the students who has not submitted the assignment by clicking on 'Broadcast' on next page.