This is how to do that the original code is by my friend Shyam and there is a more complex version on his site.
Sub GlobalFindAndReplace()' original code by Shyam Pillai
' more complex version
' http://skp.mvps.org/ppt00025.htm#2
Dim oPres As Presentation
Dim oSld As Slide
Dim oShp As Shape
Dim FindWhat As String
Dim ReplaceWith As String
FindWhat = "ß2"
ReplaceWith = "ß" & ChrW(8322)
For Each oPres In Application.Presentations
For Each oSld In oPres.Slides
For Each oShp In oSld.Shapes
Call ReplaceText(oShp, FindWhat, ReplaceWith)
Next oShp
Next oSld
Next oPres
End Sub
Sub ReplaceText(oShp As Object, FindString As String, ReplaceString As String)
Dim oTxtRng As TextRange
Dim oTmpRng As TextRange
Dim I As Integer
If oShp.HasTextFrame Then
If oShp.TextFrame.HasText Then
Set oTxtRng = oShp.TextFrame.TextRange
Set oTmpRng = oTxtRng.Replace(FindWhat:=FindString, _
Replacewhat:=ReplaceString, WholeWords:=True)
Do While Not oTmpRng Is Nothing
Set oTmpRng = oTxtRng.Replace(FindWhat:=FindString, _
Replacewhat:=ReplaceString, _
After:=oTmpRng.Start + oTmpRng.Length, _
WholeWords:=True)
Loop
End If
End If
End Sub