To edit multiple assignment due dates and/or availability dates at once start by clicking on the "Course Admin" tab in the blue navbar.

Scroll down and click on "Assignments" under the Assessment section.

Click on the box to the left of any assignments you'd like to edit, then click on the blue "Bulk Edit" button 

From here, you can adjust the due date and availability date for each assignment in one window. That way, you do not need to click into each individual assignment.

Click on the blue "Save" button at the bottom left of the page when finished.