//Sendback a activity instance to previous step

IWorkflowService wfService = new WorkflowService();

var wfResult = wfService.CreateRunner("20", "Alice")

            .UseApp("DS-100", "Book-Order", "DS-100-LX")

            .UseProcess("PriceProcessCode")

            .PrevStepInt()

            .OnTask(id)             //TaskID

            .SendBack();