The symbol dialog panel is launched from it's icon
in the document/definitions menu and
is displayed in grab mode.
This panel is also used for symbol creation and refactoring .
With this dialog:
Copy/delete symbols,
Modify their attributes,
Select an element of a symbol,
Then set its style properties (fill, stroke)
Try the online symbol editing tool via this link .
See creating , refactoring symbols showcases for interactive demonstrations.