One way to change this to change the maximum points for the grade item. Desire2Learn can then recalculate the points for the grade item based on the quiz score:
- After you change the maximum points for the grade item, click on the Grade icon for the quiz
- Clear the graded check box for each attempt so it is marked as ungraded
- Click the Update button
- Click in the box to mark each attempt as graded
- Click the Update button

If you want to go the other direction (i.e., change the points possible on the quiz to match the maximum points for the associated grade item), the steps are a bit different:
