How to change the default button text "choose a file" on the widget?

You can text with the help of UPLOADCARE_LOCALE_TRANSLATIONS global variable. 

<script>

UPLOADCARE_PUBLIC_KEY = "demopublickey";

UPLOADCARE_LOCALE_TRANSLATIONS = {
  buttons: {
    choose: {
      files: {
        one: 'Choose a file',
        other: 'Choose files'
      },
      images: {
        one: 'Choose an image',
        other: 'Choose images'
      }
    }
  }
};
</script>

 Tweak Files settings object to put a custom text in a single or multiple widget buttons.  Images settings object will overshadow Files if images-only attribute is set for a widget.