Private Sub Worksheet_BeforeDoubleClick(ByVal Target As Range, Cancel As Boolean)
With Target
    If .Column =  1  Then
        Select Case .Value
               Case Is = "a"
                    .Value = "r"
                    .Font.Name = "Marlett"
                   
                Case Is = "r"
                    .Value = "a"
                    .Font.Name = "Marlett"
                   
                Case Else
                    .Value = "a"
                    .Font.Name = "Marlett"
                   
        End Select
    End If
End With

End Sub