echo '<td><input type="checkbox" name="selected_rows[]" value="' . $row['leden_id'] . '"></td>';

$sql = $db->prepare("UPDATE `leden_tbl` SET `leden_beurten_left` = `leden_beurten_left`-1 WHERE `leden_id` = :rowid");
foreach ($selected_rows as $row_id) {
    $sql->execute([':rowid' => $row_id]);
}
$sql->closeCursor();

<form method="post" action="update_rows.php">
  <table>
    <thead>
      <tr>
        <th>Select</th>
        <th>Naam</th>
        <th>Beurten</th>
      </tr>
    </thead>
    <tbody>
      <?php
        // Connect to the MySQL database
        $db = new mysqli('***knip***');

        // Retrieve the rows from the database
        $result = $db->query("SELECT leden_id, leden_naam, leden_beurten_left FROM leden_tbl ORDER BY leden_naam ASC");

        // Loop through the rows and create a checkbox for each one
        while ($row = $result->fetch_assoc()) {
          echo '<tr>';
          echo '<td><input type="checkbox" name="selected_rows[]" value="' . '"></td>';
          echo '<td>' . $row['leden_naam'] . '</td>';
          echo '<td>' . $row['leden_beurten_left'] . '</td>';
          echo '</tr>';
        }
      ?>
    </tbody>

<?php
  // Connect to the MySQL database
  $db = new mysqli('l***knip***');

  // Retrieve the selected rows from the form data
  $selected_rows = $_POST['selected_rows'];

  // Loop through the selected rows and update the desired column
  foreach ($selected_rows as $row_id) {
    $db->query("UPDATE leden_tbl SET leden_beurten_left = leden_beurten_left -1 WHERE id = $row_id$");
  }

  // Display a confirmation message
  echo count($selected_rows) . " row(s) have been updated.";
?>