I'm really not an Excel coder but if you put this in the ThisWorkbook where the linked data is it should update the current PPT (even in show mode). Make sure you change the slide and shape number to match what you have or it will error.
Code:
Private Sub Workbook_SheetChange(ByVal Sh As Object, ByVal Target As Range)
Dim PPTApp As Object
Dim PPTPres As Object
Set PPTApp = CreateObject(Class:="PowerPoint.Application")
Set PPTPres = PPTApp.ActivePresentation
' Change to match linked shape
PPTPres.Slides(1).Shapes(3).LinkFormat.Update
End Sub