Before any change of your site like change wordpress logo code.First I strongly recommend creating a child theme for your current main theme you used.

Now copy the header.php of the main theme (currently used theme) into the child theme and open it with any code editor like notepad++ and do the following. The code may different for different theme.

<div id=”logo”>

<?php if (is_home()) { ?><h1><?php } ?>

<a class=”blog-title” href=”<?php echo home_url(); ?>/”><?php bloginfo(‘name’); ?></a>

<?php if (is_home()) { ?></h1><?php } ?>

</div>

Find this part of code <?php bloginfo( ‘name’ ); ?> and replace this line of code you’re your logo image.

For Example: <img href=“http://www.yourdoamin.com/upload/2014/05/logo.png”; >

Also you can add additional information like the bellow code instead of the above image code.

<img title=“Your Title” alt=“Description of your site logo” href=“http://www.yourdoamin.com/upload/2014/05/logo.png”; >