Changing custom header image social

On my home page header I have three social links that a web developer installed years ago when the site was made. Facebook, Twitter, and Google Plus. With Google plus no longer relevant, I was able to update the code to link that to our Instagram page. When I did that, it changed the image from the google “g+” logo to the facebook logo.

Where to I add the instagram image in my version of WordPress? I could not find it in Media or in any of the css. files.

I think this is a very simple fix, I just don’t know where the directory is that has the following images:

<div id=”social-links” class=”top-social-links”>

The page I need help with: [log in to see the link]