This method is used to update an existing entity data based on the primary key or an alternate key. The following is some sample code that can be used to update data based on the primary key:

Entity vehicle = new Entity("him_vehicle");
vehicle.Id = new Guid("E8D74065-C38D-E911-A969-000D3A29FEA9");
vehicle["him_vehicletype"] = new OptionSetValue(910600005);
OrgService.Update(vehicle);

In the preceding code, we are updating the vehicle entity record based on the GUID. You can refer to http://himbap.com/blog/?p=1522 to learn how to update data based on an alternate key.