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