Once an assignment is created you can edit the due date, or unassign it, by viewing the details of an assignment and then clicking "Edit". (See how to create assignments here.)
There you can edit the due date or remove an assignment. Please also note that if a student has already started one of the activities, you will not be able to un-assign or update it. Once an assignment is unchecked as pictured below, students will not be able to complete it. Students are able to submit assignments up until 11:59pm on the selected due date.