How do I add a logo to my website?

The title is probably placed within the header.php file.

So openup and find something like:

<h1><a href="https://wordpress.stackexchange.com/questions/187487/<?php bloginfo("url');?>"><?php bloginfo('name');?></a></h1>

This is what displays Abraham & Co on your website.

Now first of you need your image uploaded to you images folder in the theme folder.
Add the following code BEFORE the code above:

<img src="https://wordpress.stackexchange.com/questions/187487/<?php bloginfo("template_directory');?>/images/your-logo.png">

make sure you change your-logo.png to whatever image file you have uploaded to you images folder.

Optionally you can wrap the image in a div with a class to add some css in your style.css file

— UPDATE —
I have downloaded the theme that you use but indeed this isn’t in the header.php file. There is a file called template-tags.php.
In this file on rule 17 you find <h1 class="site-title"><?php bloginfo('name'); ?></h1>

This is what displays Abraham & Co on your website. But you should first check to add a logo via the admin screen options of you theme because I believe you can just upload an image.

If you can’t do the following:
Upload your image to your images folder in your theme folder.
Replace:

<a href="https://wordpress.stackexchange.com/questions/187487/<?php echo esc_url(home_url("https://wordpress.stackexchange.com/")); ?>" title="<?php echo esc_attr( get_bloginfo( 'name', 'display' ) ); ?>" rel="home">
    <h1 class="site-title"><?php bloginfo('name'); ?></h1>
</a>

and replace it with:

<img src="https://wordpress.stackexchange.com/questions/187487/<?php bloginfo("template_directory');?>/images/your-logo.png">
<a href="https://wordpress.stackexchange.com/questions/187487/<?php echo esc_url(home_url("https://wordpress.stackexchange.com/")); ?>" title="<?php echo esc_attr( get_bloginfo( 'name', 'display' ) ); ?>" rel="home">
    <h1 class="site-title"><?php bloginfo('name'); ?></h1>
</a>