Assuming the machine name of your content type is "test".
Please navigate to:
admin/structure/types/manage/test/form-display
You should see the following form (with different fields, of course).
In my example, the name of the field is simply "date" . You can change the widget to Select list and then you will see the cog wheel on the right side of the row.
After clicking on it, you shall be able to choose your preferred Date Format.
Hopefully that helps.