Editing a theme

If you have no coding experience the easiest would be to add a text widget in the header section and use an image of the button and make it link to where you want it to go. Just upload the image to your media and then check the URL of the image in media.

Something like this:

<a href="https://wordpress.stackexchange.com/questions/191039/landingpageURL">
  <img src="buton.jpg" alt="Button alt text" style="width:42px;height:42px;border:0;">
</a>