Sub bb()
Dim s As Series, i&
For i = 1 To ActiveSheet.ChartObjects.Count
    For Each s In ActiveSheet.ChartObjects(i).Chart.SeriesCollection
        s.FormulaLocal = Replace(s.FormulaLocal, "Книга1.xls]Лист1", "Книга2.xls]Лист2")
    Next
Next
End Sub