Select the component representing the tab (typically a JPanel) and go to the Layout properties tab in Inspector. You can set tab title here, as well as tooltip and icon. For example, step through the following procedure.

  1. Create a new JFrame.
  2. Drop a JTabbedPane component onto the JFrame in the Form designer window.
  3. Drop a JPanel component onto the JTabbedPane. A tab will appear.
  4. In Inspector select the JPanel, then click the Layout tab for the JPanel.
  5. The Tab Icon, Tab Title, and Tab Tooltip properties will be exposed, and can be modified.

Source: NetBeans User FAQ