How to change the date format for date entity field
You can usually only change the output of a date field, as it is easier to convert an existing date to something else. 
The input is often standardised to make sure the system understands the input.

In order to change the output, create a new date format admin/config/regional/date-time/formats and enable it on admin/config/regional/date-time.
Then you can use it as a display for the field on the display tab.