Existing Rubrics cannot be modified. If you want a different rubric you need to create your own new one which you can then edit if you want.
Click New Rubric
Give the new rubric a name and then save it
Click on the new rubric name to add values
Click Add Mastery Level as many times as you want values (say four times)
You now have something as follows:
Set the following values and colors and click close to save
