Error: Undefined index

It seems that your code is not expressing your idea is to display only one record, use WHERE clause in select.

$stmt = $db->query("SELECT * FROM news WHERE id = '$newsid' ORDER BY id");

To avoid sql injection use prepared statements, the code does not change much, a step is added to the process:

$sql = 'SELECT * FROM news '; 

 if(!empty($newsid){// caso tenha um id exibe somente ele
    $sql .= ' WHERE id = ? ORDER BY id';
 }    

$stmt = $db->prepare($sql);
$stmt->execute(array($newsid)); // aqui faz o bind da interrogação com $newsid
$itens = $stmt->fetchAll(PDO::FETCH_ASSOC); //retorna todas as linhas de uma vez.

foreach($itens as $myrow){
   echo ....
}

To receive parameters through the url, you need to use the same name defined in the link, eg: yoursite.com?newsid=ID_DA_NOTICIA, you should call this

 $newsid = $_GET['newsid'];

and not $_GET['ID'] Because the string ID has not been defined in the link.

You have an error in $myrow[ID] in the three links lines that make up the HTML. The correct one must be $myrow['ID'] . And it needs to encapsulate between braces because it's a complex expression. So:

echo "<br><a href=\"testonho.php?newsid={$myrow['ID']}\">Read More...</a>
            || <a href=\"edit_news.php?newsid={$myrow['ID']}\">Edit</a>
             || <a href=\"delete_news.php?newsid={$myrow['ID']}\">Delete</a><br><hr>";

With the new information passed in commentary on how the first call is being made it becomes clear that it needs another change. Line 6 needs to be changed to:

$newsid = $_GET['newsid'];

In addition, it has a nice vulnerability that could allow SQL Injection but not related to the reported problem.