Alter the labels of the default “post” post type.

function yourtheme_alter_post_types() {

  global $wp_post_types;

  if (!empty($wp_post_types['post']->labels)) {

    $wp_post_types['post']->labels->name = 'News';
    $wp_post_types['post']->labels->singular_name = 'News Item';
    $wp_post_types['post']->labels->add_new_item = 'Add News Item';
    $wp_post_types['post']->labels->edit_item = 'Edit News Item';
    $wp_post_types['post']->labels->new_item = 'New News Item';
    $wp_post_types['post']->labels->view_item = 'View News Item';
    $wp_post_types['post']->labels->view_items = 'View News Items';
    $wp_post_types['post']->labels->search_items = 'Search News Items';
    $wp_post_types['post']->labels->not_found = 'No news items found.';
    $wp_post_types['post']->labels->not_found_in_trash = 'No news items found in bin.';
    $wp_post_types['post']->labels->all_items = 'All News Items';
    $wp_post_types['post']->labels->archives = 'News Item Archives';
    $wp_post_types['post']->labels->attributes = 'News Item Attributes';
    $wp_post_types['post']->labels->insert_into_item = 'Insert into news item';
    $wp_post_types['post']->labels->uploaded_to_this_item = 'Uploaded to this news item';
    $wp_post_types['post']->labels->filter_items_list = 'Filter news items list';
    $wp_post_types['post']->labels->items_list_navigation = 'News items list navigation';
    $wp_post_types['post']->labels->items_list = 'News items list';
    $wp_post_types['post']->labels->item_published = 'News item published.';
    $wp_post_types['post']->labels->item_published_privately = 'News item published privately.';
    $wp_post_types['post']->labels->item_reverted_to_draft = 'News item reverted to draft.';
    $wp_post_types['post']->labels->item_trashed = 'News item binned.';
    $wp_post_types['post']->labels->item_scheduled = 'News item scheduled.';
    $wp_post_types['post']->labels->item_updated = 'News item updated.';
    $wp_post_types['post']->labels->item_link = 'News item link.';
    $wp_post_types['post']->labels->item_link_description = 'A link to a news item.';
    $wp_post_types['post']->labels->menu_name = 'News Items';
    $wp_post_types['post']->labels->name_admin_bar = 'News Item';

  }

}

add_action('init', 'yourtheme_alter_post_types');