Formatting cells doesn't change the date in excel

Hi all,

I need to change the date format in excel 2010 from mm/dd/yyyy h:mm AM/PM tomm-dd-yy h:mm(24hour).

e.g 01/10/2014  3:17:00 PM to 01-10-14 15:17.  I tried formatting the cells in excel using format cells-->custom option but the date remains the same as entered.

Does any body have some suggestion or different method to do so.

Thanks and Regards

Dnivra