To make changes to an existing symbol in a document simply drag it into your document from the Library List and check the Unlock button. When a document is unlocked you can edit the individual objects that make up a symbol, or even delete them, then, when you lock the document again, the symbol will once again behave like a single object.
You can drag an edited symbol back into the Library List under the 'My Symbols' root to save it as a User Symbol