The theme in the following example is Fusion Starter. Open the page.tpl.php in the theme folder and search for the word logo.

...
<div id="header-site-info" class="header-site-info block">
  <div id="header-site-info-inner" class="header-site-info-inner inner">
    <?php if ($logo): ?>
      <div id="logo">
        <a href="<?php print check_url($front_page); ?>" title="<?php print t('Home'); ?>"><img src="<?php print $logo; ?>" alt="<?php print t('Home'); ?>" /></a>
      </div>
    <?php endif; ?>
...

Replace the string of the title and alt attributes as shown above.

Done =)

Reference: Drupal Gardens – Site Name And Logo Alt Text (Exported Only)