add_action( 'woocommerce_thankyou', 'cheque_payment_method_order_status_to_processing', 10, 1 );
function cheque_payment_method_order_status_to_processing( $order_id ) {
if ( ! $order_id )
return;

$order = wc_get_order( $order_id );

// Updating order status to processing for orders delivered with Cheque payment methods.
if ( get_post_meta($order->id, '_payment_method', true) == 'cheque' )
$order->update_status( 'processing' );
}