How to update Ticket number field of Case using code

Hello,

Iam trying to update ticket number field of CASE but it is not updating . Is there any way to update ticket number which is already generated while creating case  .

Entity objcase = new Entity("incident", entity.Id);
objcase .Attributes["ticketNumber"] = "33843483"; 

_orgService.Update(objcase );

Thanks,

Krishna Prasad.