Hi Eric,
Symbols under My Libraries can be edited directly by doubleclicking on the symbol in the Library List. The symbols we supply under Offline and Online Libraries cannot be edited in this way.
To edit Offline and Online symbols, simply drag them into the document and check the Unlock document button - this temporarily disables grouping throughout the document and allows you to change anything within the symbol, delete items from it, or add items to it. When you have changed a symbol you should drag it into a folder under My Libraries for future use.
Remember to uncheck Unlock once you have finished editing the symbol.