The reason for your problem is that Excel did not recognize dd.mm.yy as a date, and it has left it as text.
Because it is text, Excel can't display the date in different formats.
The solution is to create a date that Excel recognizes, out of the text.
If you have a date (dd.mm.yy) in cell A11 for example, put this formula in the adjacent cell:
The formula can be dragged down alongside columns of dates as required.
Copy the new date cells and use PasteSpecial - Paste Values, to paste the new Excel dates back into the cells with the original dates. Format the cells as required, e.g., dd/mm/yy