As long as the course result is unratified you can change marks through the Enter Marks screen for one or more students or through the little i button for an individual student.
If the course result is ratified you need to unratify this first. For guidance please see Unratify.
Change item or assessment marks
You can change item or assessment marks through either the "Enter Marks" screen or an individual student's little i button.
Change the final course result
You can change the course result for an individual student through their little i button. You can also change multiple students to selected grades from the Process Course Results screen.