Here is the process to change your Grade Scheme. You can choose from Percentage, or Letter Scheme:
Click on Grades Tool on the navigation toolbar.
Click on Schemes tab option near the top of the screen.
Click on the check mark called Percentage, or Letter Grade and it will highlight it green and change you overall Grade Scheme.
Example below shows that scheme is about to change from Percentage to a Letter Grade:
Then confirm, by clicking Yes: