Use TXDataClient.Put to update an existing object in the server.
Customer := Client.Get<TCustomer>(10);
Customer.City := 'London'; // change city
Client.Put(Customer); // send changes
Pay attention to client memory management to learn which objects you need to manually destroy. Client won't destroy objects passed to Put method. In the above example, though, the object doesn't need to be destroyed because it was previously retrieved with Get, and in this case (for objects retrieved from the server), the client will manage and destroy it.