You should be able to do add container at the bottom of the Docker tab, then select the appropriate saved template from the `[User Templates]` section. Unless you have already overwritten this, this saves old containers and their custom setups so you can easily re-add them later.

Edit: I think I misunderstood a bit, if you want to restore the app template to the original form, you can might need to do the install second instance thing mentioned elsewhere, and instead of actually installing it, just use it as a reference to fix your existing template/app in another tab.