Currently there are two ways to accomplish this. We are aware that many of you would like there to be a "re-submit" options for students available without a push from you. This has been submitted as a feature request.
Allow "re-try" while grading from your "to-do" list
You may send items back to students as you gradet them by clicking "allow re-try" .
The second way to send work back to students is to use the "Batch Update" feature in the gradebook.
Click on your gradebook tool
Click on "activities"
Click on the activity you wish to grade.
Now click "Batch Update"