In the same way that a teacher can mark a students assignment as complete he/she can also change an assignment from complete to incomplete
A teacher can do this either
within the students work by Accessing student work and going to the Education menu item within the IDE
or
selecting the assignment in the course and hovering over the students name in the list to see the switch appear to the left of the students name, which you can then flip
You can also Disable Mark as Completed for individual assignment if you wish to stop students marking specific assignment as complete in error. If you have 'playground' type apps in a course for students to experiment with, this can be particularly useful