There are a variety of things we can do here.
You can simply change the grade.
You can change the grade and add the old grade to a comment. (you have to click the expand icon to show the other options)
Notice there is a red triangle on the grade to indicate you left a comment.
Alternatively, some people want to add another assignment called retake and drop the old score.
You all know how to enter an assignment, so I will skip that.
To drop the score, expand the extra tools, and click the DR
The first four letters here are simply indicators for an assignment and have no impact on the grade, they are for you to use as a reference.
Ch is for cheated and will calculate a 0 for this assignment regardless of the score entered.
X means exempt and Dr means dropped. Both will ignore this assignment for this student regardless of the score entered. The only difference is the way they are visually displayed.