please do not edit Twenty Eleven directly but create a child theme http://codex.wordpress.org/Child_Themes to work with.
it is important to have an unedited default theme in case of serious problems. also, a child theme will help to keep any customisations when you next upgrade your wordpress version.
the blog title is done in header.php of Twenty Eleven:
<h1 id="site-title"><span><a href="<?php echo esc_url( home_url( '/' ) ); ?>" title="<?php echo esc_attr( get_bloginfo( 'name', 'display' ) ); ?>" rel="home"><?php bloginfo( 'name' ); ?></a></span></h1>
for how to have conditionally different html tags on the blog title, see for instance header.php or Twenty Ten:
<?php $heading_tag = ( is_home() || is_front_page() ) ? 'h1' : 'div'; ?>
<<?php echo $heading_tag; ?> id="site-title">
<span>
<a href="<?php echo home_url( '/' ); ?>" title="<?php echo esc_attr( get_bloginfo( 'name', 'display' ) ); ?>" rel="home"><?php bloginfo( 'name' ); ?></a>
</span>
</<?php echo $heading_tag; ?>>