You might need to experiment a little but something like this should work:
Sub fixlinks()
Dim oshp As Shape
Dim osld As Slide
' I may have misunderstood what needs changing so alter if required
Const replacethis As String = "03 Halloween\05 Grafikker\Grafikker Halloween 2015.xlsx"
Const replacewith As String = "04 Jul\05 Grafikker\Grafikker Jul 2015.xlsx"
On Error Resume Next
For Each osld In ActivePresentation.Slides
For Each oshp In osld.Shapes
If oshp.LinkFormat.SourceFullName Like "X:\Analytics and Insight\_Kunder\Tivoli\2015\Exit\*" Then
oshp.LinkFormat.SourceFullName = Replace(oshp.LinkFormat.SourceFullName, replacethis, replacewith)
oshp.LinkFormat.Update
End If
Next oshp
Next osld
End Sub