To change the behaviour of a specific date field have a look at: https://docs.mendix.com/refguide/date-picker#2-6-1-date-format
To change this for the complete app goto your project settings and select the language to change the date and time formatting there.
Erwin's tips make it possible to change how the format appears in the text.
But to change the way that the popup is set up can only be changed if you modify the widget. See https://mydemoversion8-sandbox.mxapps.io/ -> AppStoreApps -> Compare dateselectors for some examples of other widgets
here is the beginning (jquery enabled):
var year = $('.dijitCalendarSelectedYear').text();
this will put the year before the month, but you will have to do the same logic when the user click elsewhere in the widget