When an assignment opens in one of your classes, you will have until the post due date to submit the assignment for full credit. To access an assignment, click the course name on the left hand navigation panel, then select the assignment you would like to work on.
You do not have to complete the assignment in one sitting.
When the due date passes, your saved answers will be locked in and graded by the instructor. Alternatively, if you feel confident in your answers before the due date, you can click Submit Assignment. While this does not prevent you from making further changes, it lets the teacher know you are ready to be graded, and they may then grade your assignment before the due date.