You can edit the logo.png or logo-dark.png from folder app\img-assets. Also, if you open each page, you will see the tag inside <header>:

<div id="logo-container" class="logo-container">
   <h1 class="site-logo logo " id="logo">
     <a href="index.html" class="site-logo-anch">
      <img class="logo-img site-logo-img" src="img-assets/logo.png" width="40" height="40" alt="Agency" title="Agency">
     </a>
   </h1>
</div>