If you don't pass any argument to form you will get form which will add new object. If you pass argument you will get form which will edit object
So you need to pass instance of object you want to edit. Something like this should do the thing
$objectEntity = Doctrine_Core::getTable("TABLENAME")->findOneBy("id",$idOfYourObject);
$this->form = new YourFormClass($objectEntity);
Because you use $this->form
this will be available in template file as $form
variable, all you have to to is
<?= $form ?>