Hello!

Hope you are doing great! 🙂

You can make use of a plugin or you can simply add a menu item and apply some custom CSS to it. To add a menu item:
1. Go to Dashboard >> Appearance >> Menu >> Top Header Menu >> Edit
2. Choose Custom Link and Add a link to Learn more there. It will be automatically added to the menu
3. Add a CSS class in that menu item

Now, Add the CSS code to create a button using custom CSS plugin or a custom CSS of your child theme. Here is the code:

li#menu-item-4340>a>.avia-menu-text {
  background-color: orange;
  padding: 10px;
  border-radius: 5px;
  color: white;
}

You can find the ID of the menu item by right-clicking on the page and select Inspect element. http://i.imgur.com/HyPTCRg.jpg

Here is a reference URL you can check out while doing this: http://www.kriesi.at/support/topic/adding-a-button-to-my-header/

Let me know if this helps!

Regards.