Assuming your date is in A1, enter the following formula in some other cell:

=TEXT(A1,"dd/mm/yy")

You can change the format "dd/mm/yy" to fit your requirement.

If you want to keep the date in cell A1 but change it so that it acts like text instead of like a date, enter an apostophe( ' ) in front of the date.
Alternatively, you can format the cell as text before entering the date.