In the appearance list, select the appearance to copy, and drag and drop it into the target library. Alternatively, in the appearance context menu, use Add to... and select the target library.
Note: You will be notified when attempting to add or copy an asset (appearance) to a library with an asset name that already exists in the library. When you add an asset to a shared library that has the same name and different properties, you will be notified that an appearance with that name already exists and have the option to keep both or replace. If you choose to replace, all appearances referencing the asset are updated to use the new asset.