<?php
add_action( 'woocommerce_thankyou', 'wc_auto_complete_paid_order', 20, 1 );
function wc_auto_complete_paid_order( $order_id ) {
    if ( ! $order_id )
        return;

    // Get an instance of the WC_Product object
    $order = wc_get_order( $order_id );

    // No updated status for orders delivered with Bank wire, Cash on delivery and Cheque payment methods.
    if ( in_array( $order->get_payment_method(), array( 'bacs', 'cod', 'cheque', '' ) ) ) {
        return;
    // Updated status to "completed" for paid Orders with all others payment methods
    } else {
        $order->update_status( 'completed' );
    }
}