...
- Use the quiz regrade
Edit existing quiz questions and tell get Canvas to recalculate student grades.
(see What options can I use to regrade a quiz in a course?) - Adjust overall quiz score using fudge points
Manually add or remove points from the student’s overall quiz score. This feature is only available via SpeedGrader.
(see How do I use quiz fudge points in SpeedGrader?) - Add a ‘no submission’ assignment
Create an assignment worth the number of points that you need to give the students additionally for the quiz. Add the assignment into the same assignment group as the quiz. Navigate to the gradebook, mute the assignment and enter the points into the column. To input the same score for all students, set a default grade for the assignment. You may need to consider how your assignments and weightings have been organised.
(see How do I set a default grade for an assignment?)
...