Code To Copy and pase as PHP sinppet:
add_action('woocommerce_thankyou','custom_woocommerce_auto_complete_order');
function custom_woocommerce_auto_complete_order( $order_id ) {
if ( ! $order_id ) {return;}
$order = wc_get_order( $order_id );
$order->update_status( 'completed' );
}