How to change Angular UI Datepicker model value format in angular formly?

Usually, in the model you store the date in ISO format like you already have, and you format it in the view as you wish, using the date filter (documentation):

<div>{{vm.model.date | date: 'yyyy/MM/dd'}}</div>

If you anyway want to change the format in the model, inject $filter and use date filter like the following:

$filter('date')(date_value, format, timezone)

e.g:

$filter('date')(vm.model.date, 'yyyy/MM/dd');

Alternatively, if you are manipulating dates a lot, a library like moment.js can be useful.

Edit: Noticed you want to change the date model value in the concrete context of angular-formly with a datepicker component. The component needs the date model value to be in a standard format, but what you can do is to have another property with the corresponding formatted date, that you would set on change of the date input:

HTML (on the datepicker input):

ng-change="vm.setFormattedDate(dt)"

Controller:

vm.setFormattedDate = function (datetime) {
    vm.model.formattedDate = $filter('date')(datetime, 'yyyy/MM/dd');
}