What kind of library item? An image or graphic? Something you've drawn in InDesign? A block of text?
In the case of external graphics (linked to a graphic outside of InDesign), it's easy. Edit the graphic (TIFF, AI file, etc) in the original application, and it will update in InDesign.
I think ID 5.5 has an Edit > Place and Link option which can create a link between the original (parent) object and a placed example.
However, InDesign cannot create such a link using a library.
These library items are graphics drawn in InDesign to look like screen controls like buttons, text boxes, drop lists etc.
Library objects do not work like the Macromedia "symbols" that Adobe inherited in Flash and Fireworks.
I don't know how, (what properties), you want to change relative to all instances of the object, but since it is a native InDesign-drawn object, there's a good chance you can use the Object tab of the Find/Change feature to change it's appearance throughout the document.
I wanted to change the stroke of numerous instances all at once. Didn't know about the the Object tab of the Find/Change feature. Very Cool - Thanks!