Problems with UPDATE in CRUD - php

Question

I'm newbie in php and i make a simple CRUD app. Unfortunately, I'm stacked with this problem, I don't know what's wrong with my code in my update.php. When i click update in my index.php it Undefined variable. I think my value in form is wrong. Any help is appreciated.
update.php
<?php
include("connection.php");
if (isset($_POST['customerNumber'])) {
$customerNumber = $_POST['customerNumber'];
$q = "SELECT customerNumber, checkNumber, paymentDate, amount FROM payments WHERE customerNumber='$customerNumber'";
$rq = mysqli_query($conn, $q);
while ($row = mysqli_feth_assoc($rq)) {
$customerNumber = $row['customerNumber'];
$checkNumber = $row['checkNumber'];
$paymentDate = $row['paymentDate'];
$amount = $row['amount'];
}
}
?>
<!-- from the index.php update -->
<form action="update.php?customerNumber=$customerNumber" method="post">
<label>
<input type="text" name="customerNumber" value="<?php echo $row['customerNumber']; ?>" placeholder="Customer Number" required>
</label>
<label>
<input type="text" name="checkNumber" value="<?php echo $row['checkNumber']; ?>" placeholder="Check Number" required>
</label>
<label>
<input type="text" name="paymentDate" value="<?php echo $row['paymentDate']; ?>" placeholder="Payment Date" required>
</label>
<label>
<input type="number" name="amount" value="<?php echo $row['amount']; ?>" placeholder="Amount">
</label>
<input type="submit" name="submit" value="update">
</form>
<?php
include('connection.php');
if (isset($_POST['submit'])) {
$customerNumber = $_POST['customerNumber'];
$checkNumber = $_POST['checkNumber'];
$paymentDate = $_POST['paymentDate'];
$amount = $_POST['amount'];
$q = "UPDATE payments SET customerNumber='$customerNumber', checkNumber='$checkNumber', paymentDate='$paymentDate', amount='$amount' WHERE customerNumber='$customerNumber' ";
$rq = mysqli_query($conn, $q);
if($rq){
header('Location: index.php');
}else{
echo "Something went wrong";
}
}
?>