If you only see the Open Stack button in the action bar when you select slides, it's probably because the slide(s) (or template) you selected is being inherited from a higher level. Higher level slides and templates are marked with a level name. You cannot edit or delete content inherited from higher levels, unless you have access to that level.

If you have higher-level permissions, go to the higher-level menu and edit the content or template at its original level. You can accomplish this by clicking the blue bar underneath the slide.