Note: This guide applies to themes which do not already include a fixed top menu bar. If you do not see this option in the Customizer, then your theme is not currently setup with this feature.
To add an extra menu to the top of your site:
1. Go to Appearance > Customize in your dashboard as shown below:
2. Go to the Layout Options section:
3. Select "Add an extra menu to the top" from the options:
4. Then click the "Save & Publish" button at the top to save the settings.
Once this setting has been enabled, you will then be able to assign a new menu to the "Top Menu Bar" position. To do this, go to Appearance > Menus and create a new menu. Once you have created a menu, you can assign it by checking the box shown below:
If you are unsure how to create a menu and assign items to it, please see this guide.