To perform bulk grade override for all students, head to the Single View in the Grades page. Next, select from the dropdown menu the activity that you'd like to override the grades for.
Single View for Quiz 1
Alternatively, you can directly jump to single view by clicking on the pen icon in the Grader report page.
Jumping to Single View for Quiz 1
To override the grade for all students, scroll to the bottom of the page and select perform bulk insert -> 0 then click on save.
Single View for Quiz 1
To avoid doing this action for all activities and to more accurately reflect students' grades at all times, it is highly recommended to uncheck the option Exclude Empty Grades for all categories.