Formula for Changing date format 3-12-13 to format d-mm-yy.

Hi

I have a long list of dates that are fomatted as 3-12-13 that I want to format under custom date as d-mmm-yy : 12-Mar-13. I tried to do this but it does not change. I tried to change it to a number and that didn't work either. Any help is appreciated.

Thank you!!
Una