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/ClasspathContainerWizard.java
public IClasspathEntry getNewEntry() {
		return fNewEntry;
	}
	/* (non-Javadoc)
	 * @see IWizard#performFinish()
	 */
	public boolean performFinish() {
		if (fContainerPage != null) {
			if (fContainerPage.finish()) {
				fNewEntry= fContainerPage.getSelection();
				return true;
			}
		}
		return false;
	}
	/* (non-Javadoc)
	 * @see IWizard#addPages()
	 */
	public void addPages() {
		if (fPageDesc == null && fEntryToEdit != null) {
			ClasspathContainerDescriptor[] containers= ClasspathContainerDescriptor.getDescriptors();
			fPageDesc= findDescriptorPage(containers, fEntryToEdit);
		}
		if (fPageDesc != null) {
			IClasspathContainerPage containerPage= null;
			try {
				containerPage= fPageDesc.createPage();
			} catch (CoreException e) {
				handlePageCreationFailed(e);