How to alter the text of the post “Excerpt” box label in WordPress post editor?

there is a filter hook you can use and it is gettext
here:

add_filter( 'gettext', 'wpse22764_gettext', 10, 2 );
function wpse22764_gettext( $translation, $original )
{
    if ( 'Excerpt' == $original ) {
        return 'My Excerpt label';
    }else{
        $pos = strpos($original, 'Excerpts are optional hand-crafted summaries of your');
        if ($pos !== false) {
            return  'My Excerpt description';
        }
    }
    return $translation;
}