Script errors

@whiteyoh — the behaviour described matches three common mistakes: the SELECTed row is fetched but never used to populate the form on first load, the hidden id field can be empty so the UPDATE's WHERE matches nothing, and a successful mysql_query call can still affect zero rows (so you need to check the affected-rows count and any DB error output). @Menster was right to flag the unused $row, and @Phaelax's request for errors is exactly what to do next. @JoshConnerty's "or die" tip is useful while debugging, but a cleaner approach is to check return values and report the specific error and affected rows.

Quick debug checklist

• Ensure a working DB connection and check the SELECT result immediately; if it fails print/log the error.
• Populate form values from the fetched row (or assign variables from the row) before rendering the HTML; include the record id in a hidden field so the UPDATE has a target. Use htmlspecialchars when echoing into value attributes to avoid breaking the HTML.
• On POST, validate/sanitize inputs and run the UPDATE. Check the query result and also check the number of affected rows — 0 means nothing changed (either WHERE mismatched or values were identical). If debugging, log the final SQL string (with care for sensitive data) or the DB error text.
• After a successful update, re-query to repopulate the form or redirect (Post/Redirect/Get) to avoid duplicate submissions.

A concise, safer pattern (use mysqli with prepared statements rather than old mysql_* calls):

$mysqli = new mysqli('host','user','pass','db');
if ($mysqli->connect_errno) die('Connect error: '.$mysqli->connect_error);

$stmt = $mysqli->prepare('SELECT Title, Subtitle, Content, Author, Date FROM indexinfo WHERE id=?');
$stmt->bind_param('i',$id);
$stmt->execute();
$stmt->bind_result($title,$subtitle,$content,$author,$date);
$stmt->fetch();
$stmt->close();

if ($_SERVER['REQUEST_METHOD']==='POST') {
  // validate/trim $_POST values, then:
  $stmt = $mysqli->prepare('UPDATE indexinfo SET Title=?, Subtitle=?, Content=?, Author=?, Date=? WHERE id=?');
  $stmt->bind_param('sssssi',$title,$subtitle,$content,$author,$date,$id);
  $stmt->execute();
  if ($stmt->affected_rows>0) { /* success */ }
  $stmt->close();
  header('Location: '.$_SERVER['REQUEST_URI']); exit;
}

Final notes: migrate off deprecated mysql_* functions (use mysqli or PDO), always escape output with htmlspecialchars, prefer prepared statements to prevent injection, and avoid using ambiguous column names like Date where possible.