how to update task status from custom task in state machine workflow

i m using custom task for state machine workflow

i am using

if(this.onTaskChangedSupervisorAction_AfterProperties.ExtendedProperties["Workflowstatus"].ToString().Contains("Approved"))

if above is true moving forward its working fine but i want to update the status field of task as i can update the title and due date for the task but how can i change the status once approved it should show the status as approved it's only showing in progress

this.CreateTaskSupervisorAction_TaskProperties.Title="supervisor task"

MCTS,ITIL