If you want to override a template coming with an app in django (in app/templates/app/) you create a template of the same name in another directory, which the template loade
One simple way to do it is this:
Let's say you want to extend and override django/contrib/admin/templates/admin/change_form.html.
First, copy the original change_form.html to your app's template directory and rename it as something like myapp/templates/admin/original_change_form.html. (You could also do this as a symlink.)
Second, create your own change_form.html in myapp/templates/admin. At the top of it, put the following:
{% extends "admin/original_change_form.html" %}
Simple!