Click assignment title in gradebook header
In the gradebook, select the assignment for which you would like to curve grades (in this example, a Quiz).
View "Numbers" and "Frequency of Grades"
Scroll down the assignment detail until you get to the Numbers and Frequency of Grades. In the Numbers section, you can see average and high scores. This can help you determine a new max score to set for your curve.
Note the grade distribution in Frequency of Grades. This distribution will shift to higher scores once you apply your curve.