If your course is an active self-paced course and you have missed the suggested due date on an assignment, you can shift your due dates into the future yourself. You can read more about self-paced courses in this article.
If you have missed a suggested due date, you can find the Shift due dates button on the course homepage or on the Dates tab.
It is not possible to shift a suggested due date before it has passed. You will only see the Shift due dates button if you have missed a suggested due date, not before.
It is also not possible to shift the final course end date or due dates on open response assessments as both of those are fixed by the course management.
If your course is an instructor-paced course, the due dates are fixed. Your course staff may be able to make an exception to provide a short extension for assignments. Please reach out to your course team directly with any contact information they provide within the course, or post on the course discussion forum.
It is certainly possible to pass an edX course if you miss some assignments; however, coursework is progressive, so you should review and study what you may have missed.
Some courses automatically drop one or more of your lowest graded assignments. Check your course syllabus for important dates and more information.