How to rename wordpress Widget

The classes assigned to rendered Widgets are specified by the argument array passed to the register_sidebar() call used to register the sidebar. e.g.:

<?php 
    $args = array(
        'name'          => sprintf(__('Sidebar %d'), $i ),
        'id'            => 'sidebar-$i',
        'description'   => '',
        'before_widget' => '<li id="%1$s" class="widget %2$s">',
        'after_widget'  => '</li>',
        'before_title'  => '<h2 class="widgettitle">',
        'after_title'   => '</h2>' ); 
?>

See the before_widget parameter:

'before_widget' => '<li id="%1$s" class="widget %2$s">'

…change it accordingly, e.g. to:

'before_widget' => '<li id="%1$s" class="block %2$s">'