<?php

/**
 * Change a field's settings when localized to the page.
 *
 * @param array $field [ id, settings => [ type, key, label, etc. ] ]
 * @return array $field
 */
add_filter( 'ninja_forms_localize_field', function( $field ){

  if( 'my_field' == $field[ 'settings' ][ 'key' ] ) {
    // Change the label setting of the field.
    $field[ 'settings' ][ 'label' ] = 'New Field Label';
  }
  return $field;
});