Locate the relevant Marksheet via Tools>Performance>Assessment>Wizard Manager.
Open the Marksheet and right click on the cell with the incorrect grade. This will allow you to view/edit the result history as per the screenshot below (Click on the image for a larger version). Highlight the grade to be removed and click the delete icon (red cross) in the top right corner. This will allow you to enter the correct grade on the Marksheet, calculate and save.
Do not overtype the cell with the incorrect grade as this will retain the previous grade in the pupil’s history and not the correct one.