Hi there!
The answer depends on whether you made the library part or it came from a managed library for which you are not the owner ... if the part came from a library that you do not own, you cannot change it .. however, if you really want to, you can make a new library part from an existing one in your own library after importing the existing one ... let me know a bit more information and i can help you get there. Right click on the part in question and Choose "Properties" and you can view what library it comes from.
