Updating specific data in MySql table through form - php

Question

I am making a online application used for storing user data and order information. I have successfully managed to create a system to which you first add customers and then add orders to specific customers.
I now want to create a function that would let the administrator of the page edit a certain line in the table.
I managed to figure out how to manually change it through a MySql code but I cannot figure out how to change it through a form? Right now I have this, but I do not think this is the correct way to do it... It works, but is very limited. For example, I am not sure what to do, if admin selects date?
PHP AND HTML
<?php
$id = $_GET['id'];
?>
<form action="update.php" method="POST" >
<input type="text" name="id" style="display:none;" value="<?php echo $id; ?> placeholder="Hidden id">
</select>
<select name="change"style="width:32.9%;" required>
<option selected hidden value="">Select a entry you want to edit</option>
<option value="narocnik">Customer</option>
<option value="naslov">Adress</option>
<option value="kraj">City</option>
<option value="enaslov">E-mail</option>
<option value="telefon">Number</option>
<option value="datumzacetka">Start date</option>
<option value="datumkonca">due date</option>
<option value="material">Material</option>
</select>
<input type="text" name="new" style="width:32.5%;" placeholder="New entry">
<input type="submit" value="Pošlji">
</form>
AND THE MYSQL PART
<?php
include 'baza.php';
$change = $_POST['change'];
$new = $_POST['new'];
$id = $_GET['id'];
$id = $_POST['id'];
$sql = "UPDATE projekti SET $change='$new' where id='$id'";
if ($conn->query($sql) === TRUE) {
header("Location: projekti.php?id=$id");
} else {
echo "Error updating record: " . $conn->error;
}
$conn->close();
?>
As I said, the function works, but it is far from perfect, so any suggestions would be very welcome :) The most important thing would be the ability to have all the fields displayed but only update the newly inputted ones.