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
for (int i= 0; i < cplist.size(); i++) {
			CPListElement elem= (CPListElement)cplist.get(i);
			if (elem.getEntryKind() == IClasspathEntry.CPE_LIBRARY) {
				IResource resource= elem.getResource();
				if (resource instanceof IFile) {
					res.add(resource);
				}
			}
		}
		return (IFile[]) res.toArray(new IFile[res.size()]);
	}	
	
	private CPListElement newCPLibraryElement(IResource res) {
		return new CPListElement(fCurrJProject, IClasspathEntry.CPE_LIBRARY, res.getFullPath(), res);
	};
	
	private CPListElement[] chooseExtJarFiles() {
		String lastUsedPath= fDialogSettings.get(IUIConstants.DIALOGSTORE_LASTEXTJAR);
		if (lastUsedPath == null) {
			lastUsedPath= ""; 
		}
		FileDialog dialog= new FileDialog(getShell(), SWT.MULTI);
		dialog.setText(NewWizardMessages.getString("LibrariesWorkbookPage.ExtJARArchiveDialog.title")); 		
		dialog.setFilterExtensions(new String[] {"*.jar;*.zip"}); 
		dialog.setFilterPath(lastUsedPath);
		String res= dialog.open();
		if (res == null) {
			return null;
		}
		String[] fileNames= dialog.getFileNames();