Enter the students’ "section" or "recitation" field filled in (in the Classlist Editor) so there is an easy way to separate them. It doesn't matter which.
Then click "Instructor Tools". You can filter to one particular group of students by section or recitation click on the section or recitation in the filter window, then click the “change display setting" button. That gives a list of the students in the section you wish. , Highlight them all. In the right-hand window, you can click on one set(assignment), then click the button towards the bottom of the page that says "Edit one set for users" and then you can change the due date for that group of students.
Thanks - I edited the instructions for my instructors as