Bug 4240 Can't edit class path entry (1GKEPIF)
There is no way to edit an existing entry for a library on the Java Build Path project properties page (other than editing the .classpath file directly). NOTES: EG (9/22/2001 11:42:11 AM) this is particularly annoying for variables. we should have an Edit button when a variable is selected.
org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/wizards/buildpaths/LibrariesWorkbookPage.java
CPListElement selElement= (CPListElement) selElements.get(0);				
			SourceAttachmentDialog dialog= new SourceAttachmentDialog(getShell(), fWorkspaceRoot, selElement.getClasspathEntry());
			if (dialog.open() == dialog.OK) {
				selElement.setSourceAttachment(dialog.getSourceAttachmentPath(), dialog.getSourceAttachmentRootPath());
				fLibrariesList.refresh();
				fClassPathList.refresh();
			}
			break;
		}
		if (libentries != null) {
			int nElementsChosen= libentries.length;					
			
			List cplist= fLibrariesList.getElements();
			List elementsToAdd= new ArrayList(nElementsChosen);
			
			for (int i= 0; i < nElementsChosen; i++) {
				CPListElement curr= libentries[i];
				if (!cplist.contains(curr) && !elementsToAdd.contains(curr)) {
					elementsToAdd.add(curr);
					addAttachmentsFromExistingLibs(curr);
				}
			}
			fLibrariesList.addElements(elementsToAdd);
			fLibrariesList.postSetSelection(new StructuredSelection(libentries));
		}
	}
	
	private void libaryPageSelectionChanged(DialogField field) {
		List selElements= fLibrariesList.getSelectedElements();
		fLibrariesList.enableButton(IDX_EDIT, canDoSourceAttachment(selElements));