First code that I wrote used SetState message to Cancel change state of workflow instance but during first lunch I’ve got an exception that SetState message is not applicable for AsyncOperation entity. I went to SDK and found following explanation:
Updates an asynchronous operation (system job). Typically, you would update only the AsyncOperation.StateCode or AsyncOperation.PostponeUntil attributes of the asynchronous operation (system job). You can also call the IOrganizationService.Update method.
So I rewrote my code using Update message:

Entity operation = new Entity("asyncoperation")
{
    Id = <Put identifier of Workflow Instance here>
};

operation["statecode"] = new OptionSetValue(3);
operation["statuscode"] = new OptionSetValue(32);

organizationservice.Update(operation);

UPD: Here is the code that allows to postpone asyncoperation till some date:

Entity operation = new Entity("asyncoperation")
{
    Id = <Put identifier of Workflow Instance here>
};

operation["postponeuntil"] = DateTime.Now.AddDays(1);

operation["statecode"] = new OptionSetValue(1);
operation["statuscode"] = new OptionSetValue(10);

organizationservice.Update(operation);