Go to the top Right corner of your Wattle site and click Turn editing on.
Click on the Navigation tab on the left side of the screen and Select Grades
This will take you to the Grader Report where you will see your students listed, with columns of grades for each assignment.
The assignments that have been graded will be highlighted in yellow. Go to the grade you wish to edit for each student and click on it. Make the edit and hit "Enter" and this will save the altered grade.
The new grade now appears.