In the Grader report, navigate to the dropdown and select the "Grade letters" option.

Grader report
Grade history
Outcomes report
Overview report
Single view
User report
Gradebook setup
Course grade settings
Preferences: Grader report
Scales
Outcomes
Grade letters
Import
Export

Click the "Override site defaults" field and ensure it is ticked.

Make any letter or grade adjustments. The following Grade letters and their grades are the default values. 

Override site defaults

Grade letter 1

Grade letter 1
Grade letter 1
≥
Letter grade boundary 1
%

Grade letter 2

Grade letter 2
Grade letter 2
≥
Letter grade boundary 2
%

Grade letter 3

Grade letter 3
Grade letter 3
≥
Letter grade boundary 3
%

Grade letter 4

Grade letter 4
Grade letter 4
≥
Letter grade boundary 4
%

Grade letter 5

Grade letter 5
Grade letter 5
≥
Letter grade boundary 5
%

Grade letter 6

Grade letter 6
Grade letter 6
≥
Letter grade boundary 6
%

Grade letter 7

Grade letter 7
Grade letter 7
≥
Letter grade boundary 7
%

Grade letter 8

Grade letter 8
Grade letter 8
≥
Letter grade boundary 8
%

Customise Grade Letters