I am getting an undefined array key message in the update page – how to solve this error? [duplicate]

In the update page when I click the update button this error is showing:

Warning: Undefined array key “updateId”

How to solve this? Here is the code:

<?php
include('connect.php');

//id geting from the url

$id = $_GET['updateId'] ;

$sql = "SELECT * FROM seriescurd WHERE id=$id";
$result = mysqli_query($conn,$sql);
$row = mysqli_fetch_assoc($result);
// echo $row['fname'];

///show whatever the data  come from the database
$f_name = $row['fname'];
$l_name = $row['lname'];
$email = $row['email'];
$mobile = $row['mobile'];

//show updated data
if (isset($_POST['update'])) {
    $f_name = $_POST['f_name']; 
    $l_name = $_POST['l_name'];  
    $email = $_POST['email'];
    $mobile = $_POST['mobile'];
    $sql = "UPDATE `seriescurd` SET `fname`='$f_name',`lname`='$l_name',`email`='$email',
    `mobile`='$mobile' WHERE id='$id'";

    $result = mysqli_query($conn, $sql);
    if ($result) {
        echo "data updated";
    } else {
        die(mysqli_error($conn));
    }
}
   <div class="container my-5">
        <form action="">
            <div class=" form-group mb-3">
                <label>First name</label>
                <input type="text" class="form-control" name="f_name" autocomplete="off" value="<?php echo $f_name ?>">
            </div>
            <div class=" form-group mb-3">
                <label>Last name</label>
                <input type="text" class="form-control" name="l_name" autocomplete="off" value="<?      



php echo $l_name ?>">
            </div>
            <div class="form-group mb-3">
                <label>Email</label>
                <input type="email" class="form-control" name="email" autocomplete="off" value="<?php echo   $email ?>">
            </div>
            <div class=" form-group mb-3">
                <label>mobile</label>
                <input type="text" class="form-control" name="mobile" autocomplete="off"value="<?php echo   $email ?>">
            </div> 
            <button type="submit" class="btn btn-dark btn-lg" name="update">update</button>
        </form>
    </div>

This is the update page code in the index page:

<a href="update.php?updateId=">update</a>