Within the Moodle Gradebook it is possible to change the display of students results from Marks to Letter grades. 

Note: Letter Grades must be defined in advance and must be mapped to accurate numerical percentages to ensure grades are correct. MIC Moodle uses the following pre-defined set of Grade Bands:

 Grade Letter
 From (Lower)
 To (Upper)
 A1
 90.0%
 100.0%
 A2
 85.0%
 89.9%
 B1
 80.0%
 84.9%
 B2
 75.0%
 79.9%
 B3
 70.0%
 74.9%
 C1
 65.0%
 69.9%
 C2
 60.0%
 64.9%
 C3
 55.0%
 59.9%
 D1
 50.0%
 54.9%
 D2
 45.0%
 49.9%
 F
 0.0%
44.9%

If these grade letters and grading bands are not compatible with your requirements, please follow guidance in our knowledge article How do I configure Letter Grades in Gradebook? for setup of Letter Grades.    
To change the display of grades on Gradebook

1. Enter your Moodle course page and hit Enter.
The module page opens.
2. In the navigation select the Grades tab.
3. From the Grader report dropdown, Select Gradebook Setup.Grader report dropdown, highlighting the Gradebook Setup option
4. In the Gradebook setup, to change the total grade to display as Letter Grades, select the edit option opposite the module gradebook folder

edit option highlighted for module gradebook folder

You will be displayed with the module folder category settings.

5. Under Category total, click on the option to Show more...
6. Change the Display Grade Type from the Default Real to Letter. Leave all other settings as is.
Letter highlighted from dropdown of Grade display type choices
Note: Results can also be displayed in multiple ways on gradebook i.e Both Real and Letter, Percentage and Letter etc. 
7. To ensure specific assignment results show as Letter grades, follow the same steps but select the Edit  for that specific grade item.

Edit option highlighted for example assingment grade item