I have several tasks for each customer.  It is tedious to have to edit each task and repetitive to update each task.

1.  You should be able to update the due date of the task right here on the screen.  see photos

2.  I used to use a software (teamwork.com).  They allow you to update several tasks at once by increasing all task about a specific amount of days.  See photos.  In other words, if all your tasks due dates need to be moved by 3 days then you can move all tasks by 3 days.  This is so much better.