trigger LeadTaskTrigger on Lead (after update) {
    
    Set<Id> leadIds = new Set<Id>();
    
    for(Lead ld : Trigger.new){
        if(ld.Status == 'Closed - Converted'){
            leadIds.add(ld.Id);
        }
    }
    
    List<Task> leadTasks = [Select Id, Status from Task where WhoId in :leadIds];
    
    for(Task t : leadTasks){
        t.Status = 'Completed';
    }
    
    if(leadTasks.size()>0){
        UPDATE leadTasks;
        
        EmailTemplate et = [SELECT Id FROM EmailTemplate WHERE developername = 'Your_Template_Name' LIMIT 1];
        List<Messaging.SingleEmailMessage> emails = new List<Messaging.SingleEmailMessage>();
        
        for(Lead ld : Trigger.new){
            if(ld.Status == 'Closed - Converted'){
                Messaging.SingleEmailMessage singleMail = new Messaging.SingleEmailMessage();
                singleMail.setTargetObjectId(ld.Id);
                //singleMail.setPlainTextBody('Task has been automatically completed');
                singleMail.setTemplateId(et.Id);
                singleMail.setSaveAsActivity(false);
                emails.add(singleMail); 
            }
        }
        Messaging.sendEmail(emails);
    }
}