Since you have the ability to show tasks on the space calendar, the fastest way to move a number of task due dates is to drag and drop the tasks on the calendar. This is a great tip for moving an entire project out by a week or moving out tasks when the priorities have changed.
First, you will need to be displaying tasks on the space calendar. To check this setting, open the Events tab in the space. Click the gear icon in the upper right above the calendar and make sure the “include tasks” button is checked.
At this point, you can scroll to the correct calendar month to see your tasks. Then left-click on the task you want to move and hold the click. Drag the task to the new due date and release the click. Your due date will automatically be updated. Continue this process until all tasks are representing the new due dates.