Library Items can be edited or configured by double-clicking on the item in the Content Panel Library view or right-click and select the context menu items 'Details...' for Resources, or 'Properties..' for Symbols.

 

When Symbols are edited the Layout Panel view changes to edit the Symbol alone. The Symbol can be edited as per normal. The Tools available depend on the object edited. For the example image shape below Tools not available are greyed out.

 

To leave the Symbol editor select the 'Close Symbol Editor' button in the Timeline (see example below). Editing a symbol in the Symbol Editor actually edits a copy of the symbol so any changes made to the Symbol are not copied back to the Library until the Symbol Editor is closed.

 

A single shape Symbol can be edited but shapes can not be added. It is possible to add to a group or Movie Clip Symbol.

 

A shape Symbol can not be moved in the Symbol editor, though it's vertices can be changed. Note the reference point of the shape is fixed in the Symbol editor view. When editing a Movie Clip Symbol which has a shape inside it, then that shape can be moved within the Movie Clip.

 

Tip: To hole punch an existing shape Symbol convert it to a Movie Clip Symbol, apply the changes and then ungroup back to a shape.