functions.php

<?php

/**
 * State Dropdown
 * @author Bill Erickson
 * @see http://www.billerickson.net/code/wpforms-state-dropdown/
 *
 * @param array $field
 * @param array $field_atts
 * @param array $form_data
 * @return array $field
 */
function ea_state_dropdown( $field, $field_atts, $form_data ) {

    if( ! in_array( 'state', $field_atts['field_class'] ) )
        return $field;

    $field['choices'] = array();
    $states = wpforms_us_states();
    foreach( $states as $value => $label )
        $field['choices'][] = array( 'label' => $label, 'value' => $value );

    return $field;
}
add_filter( 'wpforms_select_field_display', 'ea_state_dropdown', 10, 3 );