1. Open header.php

 

2. Replace this code

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<h1 class="site-title"><a href="<?php echo esc_url(home_url('/')); ?>" rel="home"><?php bloginfo('name'); ?></a></h1>

with this

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<h1 class="logo"><a href="yourdomain.com"><img src="logo.png" alt="site name" /></a></h1>
Reference