I had tried with logic as below can any one help to correct the logic.

Code

List<Opportunity> opplist = [Select id from opportunity where StageName =:'Prospecting' and StageName =:'Closed Lost' Limit 2];

List<Task> listtask = [Select id,whatId from Task where Id IN :opplist.Id];

if(trigger.isupdate||trigger.isdelete){

if(Task task : Trigger.old)

 if(o.StageName == 'ClosedLost'){
            task.Status == 'Completed';
            listtask.add(task);
            }

      update listtask;

}