obook 5

You will not be able to edit the due date on assigned work. 

Instead, you will need to access the Assigned Work tab of the book you are using and delete the assignment by clicking the bin icon shown below. 

You can then reassign the resource/s with a new due date.

obook 4 and earlier 

Unfortunately, if you use any products that run on earlier versions of obook, you will not be able to delete or edit the details of your assessments as this feature was only built into the most recent iteration of obook. 

If you are unsure about what version your resource runs on, check the URL once your obook is open. The version will be the first component of the URL.