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
public NewVariableEntryDialog(Shell parent, String variableSelection) {
		super(parent);
		fVariableBlock= new VariableBlock(false, variableSelection);
		fResultPaths= null;
	}
	
	/* (non-Javadoc)
	 * @see Window#configureShell(Shell)
	 */
	protected void configureShell(Shell shell) {
		shell.setText(NewWizardMessages.getString("NewVariableEntryDialog.title"));  
		super.configureShell(shell);
	}	
			
	protected Control createDialogArea(Composite parent) {
		VariableSelectionListener listener= new VariableSelectionListener();
		
		Composite composite= (Composite)super.createDialogArea(parent);
		fVariableBlock.createContents(composite);
		fVariableBlock.addDoubleClickListener(listener);
		fVariableBlock.addSelectionChangedListener(listener);
		return composite;
	}
	
	/**
	 * @see Dialog#createButtonsForButtonBar(Composite)
	 */
	protected void createButtonsForButtonBar(Composite parent) {
		fOkButton= createButton(parent, IDialogConstants.OK_ID, NewWizardMessages.getString("NewVariableEntryDialog.add.button"), true);