The common way to create a Flash symbol is to draw something, then convert it.
To convert an object into a Flash Symbol, select it, and then press F8.
You are still on the stage.
The new symbol appears in the library.
The object that became a symbol now has a thin blue line around it to show it’s a symbol.
If you want to “Un-Symbol” an object – select it and hit CTRL+B,
-OR- go to Modify – Break Apart.
Repeat as necessary if it is a nested symbol (see Exercise 6), and you wish to break it all the way to the original shape forms.
This breaks the object to its components.
BUT it does not erase the original symbol from the Library.
The Break Apart command applies only to the one instance you selected on the Stage.
To remove it from the file you will have to select it in the library pane and delete manually.
Browse the Flash FAQ for Animators
Exercise 2: Convert to Symbol + Break Apart
Exercise 3: Create Multiple Instances + Distribute to Layers
Exercise 4: Transform instances + color effects
Exercise 5: Edit Symbol + Navigate in and out of symbols
Exercise 6: nested symbols: Sun animation
Exercise 7: Duplicate + Swap