Sub example_03()    'msoFileDialogFolderPicker
Dim Fold As String, f As String
Dim cnt As Long
With Application.FileDialog(msoFileDialogFolderPicker)
    .Title = "Select the folder in which the files to be processed"
    .ButtonName = "Select": .AllowMultiSelect = False
    If .Show Then Fold = .SelectedItems(1) Else Exit Sub
End With
Application.ScreenUpdating = False

If Right(Fold, 1) <> "\" Then Fold = Fold & "\"
f = Dir(Fold & "*.xls*", vbNormal)
Do While f <> ""
    '    MsgBox f
    cnt = cnt + 1
    With Workbooks.Open(Fold & f)
        With .Sheets(1)
            .Range("B1").Value = Replace(.Range("B1").Value, "NNN", cnt)
        End With
        .Close True
    End With
    f = Dir()
Loop

Application.ScreenUpdating = True
End Sub