You can edit an assignment's due date or delete an assignment from the Assignment page. Click the three stacked dots in the corner of an assignment to see your options. You can also access these options from the Assignment Dashboard.
You cannot currently turn guided reading mode on or off after an assignment has been created. You will need to delete the assignment and create it again with guided reading mode enabled or disabled. Be sure to double check whether you click the checkbox for Guided Reading Mode before assigning a text. If you delete an assignment and create a new one, be sure to update any assignment links you may have shared with your students on Google Classroom or elsewhere. You can access a new assignment link by clicking Share Assignment form the Assignment Dashboard, or by clicking the three stacked dots in the corner of an assignment on your Assignments page.