Change order status in WooCommerce for specific payment method

Copy this code in your theme's function.php file

<?php
/* Reset status of new orders from "pending" to "completed" */
add_action( 'woocommerce_thankyou', 'changeOrderStatus' );
function changeOrderStatus($order_id) {
    global $woocommerce;
     if (!$order_id )
        return;
 $billing_paymethod = get_post_meta($order_id,'_payment_method',true);
  $order = new WC_Order( $order_id );
  /* put here your specific payment method */
 if($billing_paymethod == 'paypal')
 {
   /* put here your specific  order status */
       $order->update_status( 'completed' );
    }
    else
    {
     /* put here your specfic default order status for all payment method */
  $order->update_status( 'processing' );   
    }
}
?>