Lesson: 27 | Prevents users to change Stage Name of Won and Lost Opportunities

Apex Trigger

trigger OpportunityTrigger on Opportunity (before update) {
    for(Opportunity oppNew : Trigger.New){
        for(Opportunity oppOld : Trigger.Old){
            if(oppOld.StageName=='Closed Won' || oppOld.StageName=='Closed Lost'){
                if(oppOld.StageName!= oppNew.StageName){
                    oppNew.StageName.addError('You can not update the stage name');
                }
            }         
        }
    }
}