I gonna do a search and replace
Dim wks As Worksheet
Dim fndList As Variant
Dim rplcList As Variant
Dim x As Long
Set wks = ActiveWorkbook.ActiveSheet
fndList = Array("ä", "ö", "ü", "Ä", "Ö", "Ü", "ß")
rplcList = Array("ä", "ö", "ü", "Ä", "Ö", "Ü", "ß")
Application.ScreenUpdating = False
With wks
For x = LBound(fndList) To UBound(fndList)
ActiveSheet.Cells.Replace.Replace what:=fndList(x), replacement:=rplcList(x), lookat:=xlPart
Next x
Application.ScreenUpdating = True
Set wks = Nothing
Erase fndList
Erase rplcList