How to change date format from dd-mm-y to dd-mm-yy in formula bar of ms-excel?

On format cell you can select an option which date format  and this format cells can be found in more number options of number standard tool bar from home toolbar.

select cell and click on format cell ..

a dialog box appear..

under the number tab ...select date 

and choose your suitable option ...

First of all select cell, then remove value after, insert new value. And2nd option is on Standard button and go to Formate Cell button.