Change WC order status

functions.php

<?php

function my_change_status_function() {

 global $wp;

  if ( ! empty( $wp->query_vars['order-received'] ) ) {

    $order_id  = absint( $wp->query_vars['order-received'] );
    $order_key = isset( $_GET['key'] ) ? wc_clean( $_GET['key'] ) : '';

    if ( $order_id > 0 ) {
      $order = wc_get_order( $order_id );

      if ( $order && $order->get_order_key() === $order_key ) {
        $order->update_status('completed');
      }
    }

  }

}
add_action( 'init', 'my_change_status_function' );