Sub Loop_Exampl()
Dim Firstrow As Long
Dim Lastrow As Long
Dim Lrow As Long
Dim CalcMode As Long
Dim ViewMode As Long

With Application
    CalcMode = .Calculation
    .Calculation = xlCalculationManual
    .ScreenUpdating = False
End With

'We use the ActiveSheet but you can replace this with
'Sheets("MySheet")if you want
With ActiveSheet

    'We select the sheet so we can change the window view
    .Select

    'If you are in Page Break Preview Or Page Layout view go
    'back to normal view, we do this for speed
    ViewMode = ActiveWindow.View
    ActiveWindow.View = xlNormalView

    'Turn off Page Breaks, we do this for speed
    .DisplayPageBreaks = False

    'Set the first and last row to loop through
    Firstrow = .UsedRange.Cells(1).Row
    Lastrow = .UsedRange.Rows(.UsedRange.Rows.Count).Row

    'We loop from Lastrow to Firstrow (bottom to top)
    For Lrow = Lastrow To Firstrow Step -1

        'We check the values in the A column in this example
        With .Cells(Lrow, "B")

        With ActiveCell.Characters(Start:=2, Length:=1).Font
            .Superscript = False
                .Subscript = True

End With

End Sub