When you double-click an instance to edit it, you're actually editing the symbol itself in the Symbol Editor or Button Editor. To edit only the current instance, you need to break the link between the instance and the symbol. This permanently breaks the relationship between the two, however, and any future edits you make to the symbol won't be reflected in the former instance.

Button symbols have several convenient features that allow you to retain the symbol-instance relationship for a group of buttons, while assigning unique button text and URLs to each instance. For more information, see Editing button symbols.