You are going to need to edit the code inside of the wp-admin/admin-header.php file. Make sure you make a backup/copy of this file before making changes in case something goes wrong.
Once you have the admin-header.php file open look for the following line of code:
<img id="header-logo" src="../wp-includes/images/blank.gif" alt="" width="32" height="32" /> <h1 <?php echo $title_class ?>><a href="<?php echo trailingslashit( get_bloginfo('url') ); ?>" title="<?php _e('Visit site') ?>"><?php echo $blog_name ?> <span>← <?php _e('Visit site') ?></span></a></h1>
Within this line of code you are going to want to modify
<a href="<?php echo trailingslashit( get_bloginfo('url') );?>" title="<?php _e('Visit site') ?>">
Just change the above code inside of href=”” to whatever URL that you want the link to point to.