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/NewVariableEntryDialog.java
fExtensionButton= createButton(parent, IDialogConstants.CLIENT_ID, NewWizardMessages.getString("NewVariableEntryDialog.addextension.button"), false); 
		createButton(parent, IDialogConstants.CANCEL_ID, IDialogConstants.CANCEL_LABEL, false);
	}	
	
	
	protected void okPressed() {
		fVariableBlock.performOk();
		super.okPressed();
	}
	
	public IPath[] getResult() {
		return fResultPaths;
	}
	
	/*
 	 * @see IDoubleClickListener#doubleClick(DoubleClickEvent)
 	 */
	private void doDoubleClick() {
		List selected= fVariableBlock.getSelectedElements();
		if (!selected.isEmpty()) {
			okPressed();
		}
	}
	
	private void doSelectionChanged() {
		boolean isValidSelection= true;
		
		List selected= fVariableBlock.getSelectedElements();
		int nSelected= selected.size();