How can I modify page content in the admin panel?
In wp-admin/edit.php at line 189 you can see that the title text is grabbed from the labels of the post type: <h2><?php echo esc_html( $post_type_object->labels->name ); ?> … </h2> When you register a new post type (register_post_type) you can also define its labels. The available labels are listed in the Codex.