global class DailyLeadProcessor implements Schedulable{    
global void execute(SchedulableContext ctx){        
List<lead> newLead=new List<Lead>([Select Id,LeadSource from Lead where LeadSource!=null ]) ;        
if(!newLead.isEmpty()!=null){            
for(Lead l:newLead){                
l.LeadSource='Dreamforce';          
  }        
}    

}
}