I'm trying to update a custom field on opportunity '`stagechanged__c`' field (Date-datatype) when the standard stage changed from 
   Prospecting to Qualification.
   
   how to update date on custom field "`stagechanged__c`" field when standard stage field changes.
   
   
this is my batchjob :

            Global class OpportunityStageChanges implements 
           Database.Batchable<sObject> {
        
    
           Global Database.QueryLocator start(Database.BatchableContext BC) {
         
        String query = 'Select Id,Name,Amount,(Select ID, OldValue, NewValue 
        from OpportunityFieldHistory)from Opportunity';
        System.debug(query);
            
          return Database.getQueryLocator(query);
    
         } Global void execute(Database.BatchableContext BC,  
          
             List<Opportunity> 
        
           scope) {
      
             for(Opportunity opp : scope) {
          
    
            for(OpportunityFieldHistory opfh : opp.OpportunityFieldHistory) {
              system.debug(opfh.oldValue+'----'+opfh.NewValue );
            if(opfh.oldvalue == 'prospecting' && opfh.newvalue == 'Qualification'){
                
                //how to update the stagechanged__c custom field
            }
            
        }
    }
        
    }
     
          Global void finish(Database.BatchableContext BC) {}
    
         }   

how to update the stagechanged field ?

Thanks In Advance,
Sekhar