Giving a Letter Grade based off Cells

I am currently working on a project and cannot figure out how to solve it. The project has me assigning a letter grade based off grades that a person will input in a cell. I have written code that I think will do the job but it will only display an "F" where it should be an "A" or a "B". Any feedback or suggestions would be greatly appreciated. Below is what my code currently looks like. I am not looking for someone to do the project for me but rather I am stuck on a step and am seeking help on a solution.

EDIT
Since the code looks weird and not formatted correctly I have attached an image which shows what the code actually looks like

Sub Button29_Click()
Public Function GetLetterGrade(Score As Double) As String
Select Case Score
    Case Is < 0.5
        GetLetterGrade = "F"
    Case Is < 0.6
        GetLetterGrade = "D"
    Case Is < 0.7
        GetLetterGrade = "C"
    Case Is < 0.85
        GetLetterGrade = "B"
    Case Else
        GetLetterGrade "A"


End Select

End Function