Ther easiest way to do this is to first record a macro doing what you want.  
then you have to modify the macro so that the right data points will be
referenced.

It is hard to describe the code wtihout knowing what you want to do.

Hi,

The Macro Recorder is useful for tasks like this. Giving you an insight
into the objects and properties needed.

Sub Macro2()
'
' change the line between points and 4 to red
'
    ActiveChart.SeriesCollection(1).Select
    ActiveChart.SeriesCollection(1).Points(4).Select
    With Selection.Border
        .ColorIndex = 3
        .Weight = xlThin
        .LineStyle = xlContinuous
    End With
'
' change marker to red circle black border size 10
'
    With Selection
        .MarkerBackgroundColorIndex = 3
        .MarkerForegroundColorIndex = 1
        .MarkerStyle = xlCircle
        .MarkerSize = 10
        .Shadow = False
    End With
    ActiveChart.PlotArea.Select
End Sub

Cheers
Andy